BASICS OF CHEMISTRY

ORGANIC CHEMISTRY

To understand life as we know it, we must first understand a little bit of organic chemistry. Organic molecules contain both carbon and hydrogen. Though many organic chemicals also contain other elements, it is the carbon-hydrogen bond that defines them as organic. Organic chemistry defines life. Just as there are millions of different types of living organisms on this planet, there are millions of different organic molecules, each with different chemical and physical properties. There are organic chemicals that make up your hair, your skin, your fingernails, and so on. The diversity of organic chemicals is due to the versatility of the carbon atom. Why is carbon such a special element? Let's look at its chemistry in a little more detail.
Carbon (C) appears in the second row of the periodic table and has four bonding electrons in its valence shell. Similar to other non-metals, carbon needs eight electrons to satisfy its valence shell. Carbon therefore forms four bonds with other atoms (each bond consisting of one of carbon's electrons and one of the bonding atom's electrons). Every valence electron participates in bonding, thus a carbon atom's bonds will be distributed evenly over the atom's surface. These bonds form a tetrahedron (a pyramid with a spike at the top), as illustrated below:

Carbon forms 4 bonds
Organic chemicals get their diversity from the many different ways carbon can bond to other atoms. The simplest organic chemicals, called hydrocarbons, contain only carbon and hydrogen atoms; the simplest hydrocarbon (called methane) contains a single carbon atom bonded to four hydrogen atoms:

Methane - a carbon atom bonded to 4 hydrogen atoms
But carbon can bond to other carbon atoms in addition to hydrogen, as illustrated in the molecule ethane below:

Ethane - a carbon-carbon bond
In fact, the uniqueness of carbon comes from the fact that it can bond to itself in many different ways. Carbon atoms can form long chains:

Hexane - a 6-carbon chain
branched chains:

Isohexane - a branched-carbon chain
rings:

Cyclohexane - a ringed hydrocarbon
There appears to be almost no limit to the number of different structures that carbon can form. To add to the complexity of organic chemistry, neighboring carbon atoms can form double and triple bonds in addition to single carbon-carbon bonds:



Single bonding
Double bonding
Triple bonding
Keep in mind that each carbon atom forms four bonds. As the number of bonds between any two carbon atoms increases, the number of hydrogen atoms in the molecule decreases (as can be seen in the figures above).
Simple HydrocarbonsThe simplest hydrocarbons are those that contain only carbon and hydrogen. These simple hydrocarbons come in three varieties depending on the type of carbon-carbon bonds that occur in the molecule. Alkanes are the first class of simple hydrocarbons and contain only carbon-carbon single bonds. The alkanes are named by combining a prefix that describes the number of carbon atoms in the molecule with the root ending "ane". The names and prefixes for the first ten alkanes are given in the following table.
CarbonAtoms
Prefix
AlkaneName
ChemicalFormula
StructuralFormula
1
Meth
Methane
CH 4
CH4
2
Eth
Ethane
C2H6
CH3CH3
3
Prop
Propane
C3H8
CH3CH2CH3
4
But
Butane
C4H10
CH3CH2CH2CH3
5
Pent
Pentane
C5H12
CH3CH2CH2CH2CH3
6
Hex
Hexane
C6H14
...
7
Hept
Heptane
C7H16
8
Oct
Octane
C8H18
9
Non
Nonane
C9H20
10
Dec
Decane
C10H22
The chemical formula for any alkane is given by the expression CnH2n+2. The structural formula, shown for the first five alkanes in the table, shows each carbon atom and the elements that are attached to it. This structural formula is important when we begin to discuss more complex hydrocarbons. The simple alkanes share many properties in common. All enter into combustion reactions with oxygen to produce carbon dioxide and water vapor. In other words, many alkanes are flammable. This makes them good fuels. For example, methane is the principle component of natural gas, and butane is common lighter fluid.
CH4 + 2O2 CO2 + 2H2O
The combustion of methane
The second class of simple hydrocarbons, the alkenes, consists of molecules that contain at least one double-bonded carbon pair. Alkenes follow the same naming convention used for alkanes. A prefix (to describe the number of carbon atoms) is combined with the ending "ene" to denote an alkene. Ethene, for example is the two- carbon molecule that contains one double bond. The chemical formula for the simple alkenes follows the expression CnH2n. Because one of the carbon pairs is double bonded, simple alkenes have two fewer hydrogen atoms than alkanes.

Ethene
Alkynes are the third class of simple hydrocarbons and are molecules that contain at least one triple-bonded carbon pair. Like the alkanes and alkenes, alkynes are named by combining a prefix with the ending "yne" to denote the triple bond. The chemical formula for the simple alkynes follows the expression CnH2n-2.

Ethyne
IsomersBecause carbon can bond in so many different ways, a single molecule can have different bonding configurations. Consider the two molecules illustrated here:
C6H14

CH3CH2CH2CH2CH2CH3
C6H14

CH
3
I
CH3
CH2
CH
CH2
CH3
Both molecules have identical chemical formulas (shown in the left column); however, their structural formulas (and thus some chemical properties) are different. These two molecules are called isomers. Isomers are molecules that have the same chemical formula but different structural formulas.
Functional GroupsIn addition to carbon and hydrogen, hydrocarbons can also contain other elements. In fact, many common groups of atoms can occur within organic molecules, these groups of atoms are called functional groups. One good example is the hydroxyl functional group. The hydroxyl group consists of a single oxygen atom bound to a single hydrogen atom (-OH). The group of hydrocarbons that contain a hydroxyl functional group is called alcohols. The alcohols are named in a similar fashion to the simple hydrocarbons, a prefix is attached to a root ending (in this case "anol") that designates the alcohol. The existence of the functional group completely changes the chemical properties of the molecule. Ethane, the two-carbon alkane, is a gas at room temperature; ethanol, the two-carbon alcohol, is a liquid.

Ethanol
Ethanol, common drinking alcohol, is the active ingredient in "alcoholic" beverages such as beer and wine.
TYPES OF REACTIONS
1 Synthesis Reactions
1.1 Redox
2 Decomposition Reactions
3 Single Replacement Reactions
4 Double Replacement Reactions
4.1 Precipitation
4.2 Acid Base
5 Organic Reactions
6 Combustion
Synthesis Reactions
The general form of a synthesis reaction is A + B → AB. The most well-known example is the formation of water via the fusion of hydrogen gas and oxygen gas (and energy):
Synthesis reactions always yield one product.

[edit] Redox
Redox is an abbreviation of reduction / oxidation reactions. This is exactly what happens in a redox reaction, one species is reduced and another is oxidized. Reduction involves a gain of electrons and oxidation involves a loss, so a redox reaction is one in which electrons are transferred between species. Reactions where something is "burnt" (burning means being oxidised) are examples of redox reactions, however, oxidation reactions also occur in solution, which is very useful and forms the basis of electrochemistry.
Redox reactions are often written as two half-reactions showing the reduction and oxidation processes separately. These half-reactions are balanced (by multiplying each by a coefficient) and added together to form the full equation. When magnesium is burnt in oxygen, it loses electrons (it is oxidised). Conversely, the oxygen gains electrons from the magnesium (it is reduced).
Redox reactions will be discussed in greater detail in the redox section.

[edit] Decomposition Reactions
These are the opposite of synthesis reactions, with the format AB → A + B. One example is the electrolysis of water (passing water through electrical current) to form hydrogen gas and oxygen gas:
Just as synthesis reactions can only form one product, decomposition reactions can only start with one reactant.

[edit] Single Replacement Reactions
A component in a compound is swapped with another component, in the format AB + C → AC + B.

[edit] Double Replacement Reactions
In these reactions, two compounds swap components, in the format AB + CD → AD + CB.

[edit] Precipitation
A precipitation reaction occurs when an ionic substance comes out of solution and forms an insoluble (or slightly soluble) solid. The solid which comes out of solution is called a precipitate. This can occur when two soluble salts (ionic compounds) are mixed and form an insoluble one - the precipitate. An example is lead nitrate mixed with potassium iodide, which forms a bright yellow precipitate of lead iodide.
Note that the lead iodide is formed as a solid. The above equation is written in molecular form, which is not the best way of describing the reaction. Each of the elements really exist in solution as individual ions, not bonded to each other (as in potassium iodide crystals). If we write the above as an ionic equation, we get a much better idea of what is actually happening.
In the solution, there exist both lead and iodide ions. Because lead iodide is insoluble, they spontaneously crystallise and form the precipitate.

[edit] Acid Base
In simple terms, an acid is a substance which can lose a H+ ion (i.e. a proton) and a base is a substance which can accept a proton. When equal amounts of an acid and base react, they neutralize each other forming species which aren't as acidic or basic. For example, when hydrochloric acid (HCl) and sodium hydroxide (NaOH) react, they react to form water and sodium chloride (common salt, NaCl).
Again, we get a clearer picture of what's happening if we write an ionic equation.
Acid base reactions often happen in aqueous solution, but they can also occur in the gaseous state (and perhaps other states). Acids and bases will be discussed in much greater detail in the acids and bases section.

[edit] Organic Reactions
Organic reactions occur between organic molecules (molecules containing the element carbon). Since there are a virtually limitless number of organic molecules, the scope of organic reactions is very large. However, many of the characteristics of organic molecules are determined by functional groups - small groups of atoms which react in predictable ways.
Another key concept in organic reactions is Lewis basicity. Parts of organic molecules can be electrophillic (electron-loving) or nucleophillic (nucleus, or positive loving). Nucleophillic regions have an excess of electrons - they act as Lewis bases - whereas electrophillic areas are electron deficient and act as Lewis acids. The nucleophillic and electrophillic regions attract and react with each other (needless to say, this has inspired many terrible organic chemistry jokes).
Organic reactions are beyond the scope of this book, and are covered in more detail in Organic Chemistry.

[edit] Combustion
Combustion, better known as burning, is the combination of a substance with oxygen.
Chemical Reactivity
Organic chemistry encompasses a very large number of compounds ( many millions ), and our previous discussion and illustrations have focused on their structural characteristics. Now that we can recognize these actors ( compounds ), we turn to the roles they are inclined to play in the scientific drama staged by the multitude of chemical reactions that define organic chemistry.We begin by defining some basic terms that will be used frequently as this subject is elaborated.
Reaction Classification
Classifying Organic Chemical Reactions
If you scan any organic textbook you will encounter what appears to be a very large, often intimidating, number of reactions. These are the "tools" of a chemist, and to use these tools effectively, we must organize them in a sensible manner and look for patterns of reactivity that permit us make plausible predictions. Most of these reactions occur at special sites of reactivity known as functional groups, and these constitute one organizational scheme that helps us catalog and remember reactions.Ultimately, the best way to achieve proficiency in organic chemistry is to understand how reactions take place, and to recognize the various factors that influence their course.This is best accomplished by perceiving the reaction pathway or mechanism of a reaction.
1. Classification by Structural Change
First, we identify four broad classes of reactions based solely on the structural change occurring in the reactant molecules. This classification does not require knowledge or speculation concerning reaction paths or mechanisms.The letter R in the following illustrations is widely used as a symbol for a generic group. It may stand for simple substituents such as H– or CH3–, or for complex groups composed of many atoms of carbon and other elements.
Four Reaction Classes
Addition
Elimination
Substitution
Rearrangement
In an addition reaction the number of σ-bonds in the substrate molecule increases, usually at the expense of one or more π-bonds. The reverse is true of elimination reactions, i.e.the number of σ-bonds in the substrate decreases, and new π-bonds are often formed. Substitution reactions, as the name implies, are characterized by replacement of an atom or group (Y) by another atom or group (Z). Aside from these groups, the number of bonds does not change. A rearrangement reaction generates an isomer, and again the number of bonds normally does not change.The examples illustrated above involve simple alkyl and alkene systems, but these reaction types are general for most functional groups, including those incorporating carbon-oxygen double bonds and carbon-nitrogen double and triple bonds. Some common reactions may actually be a combination of reaction types. The reaction of an ester with ammonia to give an amide, as shown below, appears to be a substitution reaction ( Y = CH3O & Z = NH2 ); however, it is actually two reactions, an addition followed by an elimination.
The addition of water to a nitrile does not seem to fit any of the above reaction types, but it is simply a slow addition reaction followed by a rapid rearrangement, as shown in the following equation. Rapid rearrangements of this kind are called tautomerizations.
Additional examples illustrating these classes of reaction may be examined by Clicking Here
2. Classification by Reaction Type
At the beginning, it is helpful to identify some common reaction types that will surface repeatedly as the chemical behavior of different compounds is examined. This is not intended to be a complete and comprehensive list, but should set the stage for future elaborations.
Acidity and Basicity
It is useful to begin a discussion of organic chemical reactions with a review of acid-base chemistry and terminology for several reasons. First, acid-base reactions are among the simplest to recognize and understand. Second, some classes of organic compounds have distinctly acidic properties, and some other classes behave as bases, so we need to identify these aspects of their chemistry. Finally, many organic reactions are catalyzed by acids and/or bases, and although such transformations may seem complex, our understanding of how they occur often begins with the functioning of the catalyst.Organic chemists use two acid-base theories for interpreting and planning their work: the Brønsted theory and the Lewis theory.
Brønsted Theory
According to the Brønsted theory, an acid is a proton donor, and a base is a proton acceptor. In an acid-base reaction, each side of the equilibrium has an acid and a base reactant or product, and these may be neutral species or ions.
H-A + B:(–)
A:(–) + B-H
(acid1) (base1)
(base2) (acid2)
Structurally related acid-base pairs, such as {H-A and A:(–)} or {B:(–) and B-H} are called conjugate pairs. Substances that can serve as both acids and bases, such as water, are termed amphoteric.
H-Cl + H2O

Cl:(–) + H3O(+)
(acid) (base)
(base) (acid)
H3N: + H2O

NH4(+) + HO(–)
(base) (acid)
(acid) (base)
The relative strength of a group of acids (or bases) may be evaluated by measuring the extent of reaction that each group member undergoes with a common base (or acid). Water serves nicely as the common base or acid for such determinations. Thus, for an acid H-A, its strength is proportional to the extent of its reaction with the base water, which is given by the equilibrium constant Keq.
H-A + H2O
H3O(+) + A:(–)
Since these studies are generally extrapolated to high dilution, the molar concentration of water (55.5) is constant and may be eliminated from the denominator. The resulting K value is called the acidity constant, Ka. Clearly, strong acids have larger Ka's than do weaker acids. Because of the very large range of acid strengths (greater than 1040), a logarithmic scale of acidity (pKa) is normally employed. Stronger acids have smaller or more negative pKa values than do weaker acids.
Some useful principles of acid-base reactions are:• Strong acids have weak conjugate bases, and weak acids have strong conjugate bases.• Acid-base equilibria always favor the weakest acid and the weakest base.
Examples of Brønsted Acid-Base Equilibria
Acid-Base Reaction
ConjugateAcids
ConjugateBases
Ka
pKa
HBr + H2O
H3O(+) + Br(–)
HBrH3O(+)
Br(–)H2O
105
-5
CH3CO2H + H2O
H3O(+) + CH3CO2(–)
CH3CO2HH3O(+)
CH3CO2(–)H2O
1.77*10-5
4.75
C2H5OH + H2O
H3O(+) + C2H5O(–)
C2H5OHH3O(+)
C2H5O(–)H2O
10-16
16
NH3 + H2O
H3O(+) + NH2(–)
NH3H3O(+)
NH2(–)H2O
10-34
34
In all the above examples water acts as a common base. The last example ( NH3 ) cannot be measured directly in water, since the strongest base that can exist in this solvent is hydroxide ion. Consequently, the value reported here is extrapolated from measurements in much less acidic solvents, such as acetonitrile.
Since many organic reactions either take place in aqueous environments ( living cells ), or are quenched or worked-up in water, it is important to consider how a conjugate acid-base equilibrium mixture changes with pH. A simple relationship known as the Henderson-Hasselbach equation provides this information.

When the pH of an aqueous solution or mixture is equal to the pKa of an acidic component, the concentrations of the acid and base conjugate forms must be equal ( the log of 1 is 0 ). If the pH is lowered by two or more units relative to the pKa, the acid concentration will be greater than 99%. On the other hand, if the pH ( relative to pKa ) is raised by two or more units the conjugate base concentration will be over 99%. Consequently, mixtures of acidic and non-acidic compounds are easily separated by adjusting the pH of the water component in a two phase solvent extraction.For example, if a solution of benzoic acid ( pKa = 4.2 ) in benzyl alcohol ( pKa = 15 ) is dissolved in ether and shaken with an excess of 0.1 N sodium hydroxide ( pH = 13 ), the acid is completely converted to its water soluble ( ether insoluble ) sodium salt, while the alcohol is unaffected. The ether solution of the alcohol may then be separated from the water layer, and pure alcohol recovered by distillation of the volatile ether solvent. The pH of the water solution of sodium benzoate may then be lowered to 1.0 by addition of hydrochloric acid, at which point pure benzoic acid crystallizes, and may be isolated by filtration.
For a discussion of how acidity is influenced by molecular structure Click Here.
Basicity
The basicity of oxygen, nitrogen, sulfur and phosphorus compounds or ions may be treated in an analogous fashion. Thus, we may write base-acid equilibria, which define a Kb and a corresponding pKb. However, a more common procedure is to report the acidities of the conjugate acids of the bases ( these conjugate acids are often "onium" cations ). The pKa's reported for bases in this system are proportional to the base strength of the base. A useful rule here is: pKa + pKb = 14. We see this relationship in the following two equilibria:
Acid-Base Reaction
ConjugateAcids
ConjugateBases
K
pK
NH3 + H2O
NH4(+) + OH(–)
NH4(+)H2O
NH3OH(–)
Kb = 1.8*10-5
pKb = 4.74
NH4(+) + H2O
H3O(+) + NH3
NH4(+)H3O(+)
NH3H2O
Ka = 5.5*10-10
pKa = 9.25
Tables of pKa values for inorganic and organic acids ( and bases) are available in many reference books, and may be examined here by clicking on the appropriate link:
Inorganic Acidity Constants
Organic Acidity Constants
Basicity Constants
Although it is convenient and informative to express pKa values for a common solvent system (usually water), there are serious limitations for very strong and very weak acids. Thus acids that are stronger than the hydronium cation, H3O(+), and weak acids having conjugate bases stronger than hydroxide anion, OH(–), cannot be measured directly in water solution. Solvents such as acetic acid, acetonitrile and nitromethane are often used for studying very strong acids. Relative acidity measurements in these solvents may be extrapolated to water. Likewise, very weakly acidic solvents such as DMSO, acetonitrile, toluene, amines and ammonia may be used to study the acidities of very weak acids. For both these groups, the reported pKa values extrapolated to water are approximate, and many have large uncertainties. A useful table of pKa values in DMSO solution has been compiled from the work of F.G. Bordwell, and may be reached by Clicking Here.
Lewis Theory
According to the Lewis theory, an acid is an electron pair acceptor, and a base is an electron pair donor. Lewis bases are also Brønsted bases; however, many Lewis acids, such as BF3, AlCl3 and Mg2+, are not Brønsted acids. The product of a Lewis acid-base reaction, is a neutral, dipolar or charged complex, which may be a stable covalent molecule. Two examples of Lewis acid-base equilibria are shown in equations 1 & 2 below.
In the first example, an electron deficient aluminum atom bonds to a covalent chlorine atom be sharing one of its non-bonding valence electron pairs, and thus achieves an argon-like valence shell octet. Because this sharing is unilateral (chlorine contributes both electrons), both the aluminum and the chlorine have formal charges, as shown. If the carbon chlorine bond in this complex breaks with both the bonding electrons remaining with the more electronegative atom (chlorine), the carbon assumes a positive charge. We refer to such carbon species as carbocations. Carbocations are also Lewis acids, as the reverse reaction demonstrates.Many carbocations (but not all) may also function as Brønsted acids. Equation 3 illustrates this dual behavior; the Lewis acidic site is colored red and three of the nine acidic hydrogen atoms are colored orange. In its Brønsted acid role the carbocation donates a proton to the base (hydroxide anion), and is converted to a stable neutral molecule having a carbon-carbon double bond.
A terminology related to the Lewis acid-base nomenclature is often used by organic chemists. Here the term electrophile corresponds to a Lewis acid, and nucleophile corresponds to a Lewis base.Electrophile: An electron deficient atom, ion or molecule that has an affinity for an electron pair, and will bond to a base or nucleophile.Nucleophile: An atom, ion or molecule that has an electron pair that may be donated in bonding to an electrophile (or Lewis acid).
To learn more about the relationship of basicity and nucleophilicity,and for examples of acid/base catalysis of organic reactions Click Here.

Oxidation and Reduction Reactions
A parallel and independent method of characterizing organic reactions is by oxidation-reduction terminology. Carbon atoms may have any oxidation state from –4 (e.g. CH4 ) to +4 (e.g. CO2 ), depending upon their substituents. Fortunately, we need not determine the absolute oxidation state of each carbon atom in a molecule, but only the change in oxidation state of those carbons involved in a chemical transformation. To determine whether a carbon atom has undergone a redox change during a reaction we simply note any changes in the number of bonds to hydrogen and the number of bonds to more electronegative atoms such as O, N, F, Cl, Br, I, & S that has occurred. Bonds to other carbon atoms are ignored. This count should be conducted for each carbon atom undergoing any change during a reaction.
If the number of hydrogen atoms bonded to a carbon increases, and/or if the number of bonds to more electronegative atoms decreases, the carbon in question has been reduced (i.e. it is in a lower oxidation state).
If the number of hydrogen atoms bonded to a carbon decreases, and/or if the number of bonds to more electronegative atoms increases, the carbon in question has been oxidized (i.e. it is in a higher oxidation state).
If there has been no change in the number of such bonds, then the carbon in question has not changed its oxidation state. In the hydrolysis reaction of a nitrile shown above, the blue colored carbon has not changed its oxidation state.
These rules are illustrated by the following four addition reactions involving the same starting material, cyclohexene. Carbon atoms colored blue are reduced, and those colored red are oxidized. In the addition of hydrogen both carbon atoms are reduced, and the overall reaction is termed a reduction. Peracid epoxidation and addition of bromine oxidize both carbon atoms, so these are termed oxidation reactions. Addition of HBr reduces one of the double bond carbon atoms and oxidizes the other; consequently, there is no overall redox change in the substrate molecule.
For a discussion of how oxidation state numbers may be assigned to carbon atoms Click Here.
Since metals such as lithium and magnesium are less electronegative than hydrogen, their covalent bonds to carbon are polarized so that the carbon is negative (reduced) and the metal is positive (oxidized). Thus, Grignard reagent formation from an alkyl halide reduces the substituted carbon atom. In the following equation and half-reactions the carbon atom (blue) is reduced and the magnesium (magenta) is oxidized.

3. Classification by Functional Group
Functional groups are atoms or small groups of atoms (usually two to four) that exhibit a characteristic reactivity when treated with certain reagents. To view a table of the common functional groups and their class names Click Here. A particular functional group will almost always display its characteristic chemical behavior when it is present in a compound. Because of this, the discussion of organic reactions is often organized according to functional groups. The following table summarizes the general chemical behavior of the common functional groups. For reference, the alkanes provide a background of behavior in the absence of more localized functional groups.
Functional Class
Formula
Characteristic Reactions
Alkanes
C–C, C–H
Substitution (of H, commonly by Cl or Br)Combustion (conversion to CO2 & H2O)
Alkenes
C=C–C–H
AdditionSubstitution (of H)
Alkynes
C≡C–H
AdditionSubstitution (of H)
Alkyl Halides
H–C–C–X
Substitution (of X)Elimination (of HX)
Alcohols
H–C–C–O–H
Substitution (of H); Substitution (of OH)Elimination (of HOH); Oxidation (elimination of 2H)
Ethers
(α)C–O–R
Substitution (of OR); Substitution (of α–H)
Amines
C–NRH
Substitution (of H);Addition (to N); Oxidation (of N)
Benzene Ring
C6H6
Substitution (of H)
Aldehydes
(α)C–CH=O
AdditionSubstitution (of H or α–H)
Ketones
(α)C–CR=O
AdditionSubstitution (of α–H)
Carboxylic Acids
(α)C–CO2H
Substitution (of H); Substitution (of OH)Substitution (of α–H); Addition (to C=O)
Carboxylic Derivatives
(α)C–CZ=O(Z = OR, Cl, NHR, etc.)
Substitution (of Z); Substitution (of α–H)Addition (to C=O)
This table does not include any reference to rearrangement, due to the fact that such reactions are found in all functional classes, and are highly dependent on the structure of the reactant. Furthermore, a review of the overall reaction patterns presented in this table discloses only a broad and rather non-specific set of reactivity trends. This is not surprising, since the three remaining categories provide only a coarse discrimination (comparable to identifying an object as animal, vegetable or mineral). Consequently, apparent similarities may fail to reflect important differences. For example, addition reactions to C=C are significantly different from additions to C=O, and substitution reactions of C-X proceed in very different ways, depending on the hybridization state of carbon.
Reaction Variables
The Variables of Organic Reactions
In an effort to understand how and why reactions of functional groups take place in the way they do, chemists try to discover just how different molecules and ions interact with each other as they come together. To this end, it is important to consider the various properties and characteristics of a reaction that may be observed and/or measured as the reaction proceeds . The most common and useful of these are listed below:
1. Reactants and Reagents
A. Reactant Structure: Variations in the structure of the reactant may have a marked influence on the course of a reaction, even though the functional group is unchanged. Thus, reaction of 1-bromopropane with sodium cyanide proceeds smoothly to yield butanenitrile, whereas 1-bromo-2,2-dimethylpropane fails to give any product and is recovered unchanged. In contrast, both alkyl bromides form Grignard reagents (RMgBr) on reaction with magnesium.
B. Reagent Characteristics: Apparently minor changes in a reagent may lead to a significant change in the course of a reaction. For example, 2-bromopropane gives a substitution reaction with sodium methylthiolate but undergoes predominant elimination on treatment with sodium methoxide.

2. Product Selectivity
A. Regioselectivity: It is often the case that addition and elimination reactions may, in principle, proceed to more than one product. Thus 1-butene might add HBr to give either 1-bromobutane or 2-bromobutane, depending on which carbon of the double bond receives the hydrogen and which the bromine. If one possible product out of two or more is formed preferentially, the reaction is said to be regioselective.
Simple substitution reactions are not normally considered regioselective, since by definition only one constitutional product is possible. However, rearrangements are known to occur during some reactions.
B. Stereoselectivity: If the reaction products are such that stereoisomers may be formed, a reaction that yields one stereoisomer preferentially is said to be stereoselective. In the addition of bromine to cyclohexene, for example, cis and trans-1,2-dibromocyclohexane are both possible products of the addition. Since the trans-isomer is the only isolated product, this reaction is stereoselective.
C. Stereospecificity: This term is applied to cases in which stereoisomeric reactants behave differently in a given reaction. Examples include:
(i) Formation of different stereoisomeric products, as in the reaction of enantiomeric 2-bromobutane isomers with sodium methylthiolate, shown in the following diagram.
Here, the (R)-reactant gives the configurationally inverted (S)-product, and (S)-reactant produces (R)-product. The (R) and (S) notations for configuration are described in a later section of this text.
(ii) Different rates of reaction, as in the base-induced eleimination of cis & trans-4-tert-butylcyclohexyl bromide (equation 1 below).
(iii) Different reaction paths leading to different products, as in the base-induced eleimination of cis & trans-2-methylcyclohexyl bromide (equation 2 below).
The mechanisms of these substitution and elimination reactions are discussed in the alkyl halide section of this text