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α -carbon  -  The carbon adjacent to the leaving group .
β -carbon  -  The carbon(s) adjacent to the α -carbon.
β -hydrogen  -  The hydrogen(s) attached to the β -carbon.
Aprotic  -  A solvent that is not a hydrogen bond donor.
Leaving Group  -  The group that leaves in a substitution or elimination reaction.
Nucleophile  -  A group characterized by it's S N2 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.
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.