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α -carbon  -  The carbon adjacent to the leaving group .
Antiperiplanar  -  If two bonds define two line segments, then they are antiperiplanar if they are antiparallel in the plane they define. It's much easier to see antiperiplanar bonds than it is to explain them. In the following diagram, the C-H and C-LG bonds are antiperiplanar:
E2 reactions require an antiperiplanar β -hydrogen.
Aprotic  -  A solvent that is not a hydrogen bond donor.
Backside Attack  -  The SN2 mechanism in which the nucleophile attacks the α -carbon from a direction directly opposite to the C-LG bond. Results in inversion of stereochemical configuration.
β -carbon  -  The carbon(s) adjacent to the α -carbon.
β -elimination  -  A mechanism that involves the removal of a beta - hydrogen. All E2 reactions are β -eliminations.
β -hydrogen  -  The hydrogen(s) attached to the β -carbon.
Bimolecular  -  Involving two molecules. In this SparkNote, the term bimolecular is used to refer to SN2 and E2 reactions. The rate-limiting transition states of both reactions involve two molecules.
Carbocation  -  A carbon that carries a positive charge. Carbocations are highly unstable and are prone to rearrangement. SN1 and E1 reactions proceed through a common carbocation intermediate.
Concerted  -  A single step mechanism. The SN2 and E2 mechanisms are concerted.
E1  -  A reaction where a β -hydrogen is Eliminated to form a double bond. E1 reactions go through a carbocation intermediate. Somewhat similar to the E2 reaction.
E2  -  An E2 reaction in which a β -hydrogen and a leaving group are removed to form a double bond. The reaction is so named because it is bimolecular (2 molecules) and because it involves the Elimination of a β -hydrogen.
Frontside attack  -  A disproved SN2 mechanism in which the nucleophile attacks the α - carbon from the same side as the C-LG bond. If this mechanism was valid, it would result in retention of stereochemical configuration. See backside attack.
Intramolecular  -  A reaction that involves groups attached to the same molecule. Contrast with the intermolecular reaction, which takes place between groups on two different molecules.
Leaving Group  -  The group that leaves in a substitution or elimination reaction.
Nucleophile  -  A group characterized by it's SN2 reactivity. Good nucleophiles tend to be good bases, though they need not be. A nucleophile has a lone pair of electrons which makes up the business end of the molecule. See the nucleophilicity section.
Protic  -  A solvent that is a hydrogen bond donor.
Rate  -  The speed of a reaction. Often measured in moles per second.
Rate law  -  A mathematical equation that relates the rate of a reaction to the concentration of its reactants. For a generic reaction:

X + Y → Z    

The rate law is expressed as:

rate = k [X]a [Y]b    

Rate-limiting step  -  The slowest step in a reaction that determines the overall rate.
Reaction order  -  In the rate law, the value of "a" and "b" for reactants "X" and "Y," respectively. The overall order of a reaction is the sum of "a" and "b" and tells how many molecules of reactant are involved in the transition state of the rate-limiting step.
Reaction intermediate  -  An intermediary molecule that accumulates in negligible quantities during a reaction.
SN2  -  A SN2 reaction is a bimolecular (2 molecules) reaction involving the Substitution of a Nucleophile for a leaving group. See the SN2 section.
SN1  -  A reaction in which a Nucleophile Substitutes for a leaving group. SN1 . It goes through a carbocation intermediate.
Saytzeff's rule  -  Saytzeff's rule states that an E2 reaction will preferentially form the most stable alkene isomer. Alkene stability generally increases with branching at the alkene carbons.
Secondary  -  A secondary carbon has bonds to two other carbons.
Tertiary  -  A tertiary carbon has bonds to three other carbons.
Transition state  -  An extremely unstable molecule that forms between the reactants and their intermediates/products. A transition state exists at the peaks of an energy diagram, in contrast to intermediates and products, which form in the troughs. The âá symbol and brackets denote a transition state structure. The number of reactant molecules present in the rate-limiting transition state is equal to the reaction order.
Unimolecular  -  Involving one molecule. This term is used synonymously with the E1 and SN1 reactions in this SparkNote. The rate-limiting transition states of both mechanisms involve one molecule.