Organichemica
Basic Organic Chemistry

A Tutorial

Dave Woodcock ©2001

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Level 1

Glossary

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racemic mixture

A 50/50 mixture of the two enantiomers of a compound.
Reactions which form chiral products from achiral reactants (with no help from any chiral catalyst) produce racemic mixtures.

radical

An atom, either single or in an group, with an unpaired electron. Radicals are often highly reactive electron deficient species.

rate determining step

In a stepwise process, each step will have its own rate of occuring. The slowest step is the bottleneck, since the overall reaction can only occur as fast as the slowest step (or link) in the series. This slowest step therefore determines the rate of the overall process and is called the 'rate determining step', or rds.

rate of a chemical process

The rate of a chemical process is defines as the rate of production of a product with respect to time or as minus the rate of disappearance of a reactant with respect to time. Each such rate is normalized by dividing by the coefficient for the reactant or product in the balanced chemical equation. In this way the rate is the same no matter how it is measured.

reactant

A chemical substance present at the start of a chemical reaction.

reaction mechanism

A stepwise route by which the reactants are converted into the products.

reagent

A chemical added to the substrate to make a reaction proceed.

rearrangement

A process whereby the skeletal bond connections in the product differ from those in the reactant.
Carbocations are prone to rearrangements whereby a group adjacent to the positive carbon will move, with its bonding electrons, to neutralize the charge at the original carbon but creating a new positive centre at the atom from which the group migrated.

reduction

(1) A negative change in the oxidation number of an element during a reaction.
(2) The addition of hydrogen to a compound during a reaction.
(3) The removal of oxygen from a compound during a reaction.
See oxidation.

regioselective

If in a chemical process where two or more sites of reaction are possible, one is prefered over the other(s), the reaction is termed regioselective.
For example, the addition of an unsymmetrical reagent (such as H-Cl) to an unsymmetrical C=C may occur in one of two directions. The rection is regioselective if one direction is prefered over the other.

resonance

A badly named part of valence bond theory which explains the extra stability of certain atomic groupings by suggesting that the actual bonding is a compromise of the bonding described by Kekulé structures.
The bonding conditions necessary (but not defining) for this phenomenon are the presence of p atomic orbitals on three or more adjacent atoms bonded together by sigma bonds.
Since Kekulé structures do not represent the theorized bonding structure, two or more Kekulé structures are drawn connected by a double-headed arrow. Thus for benzene:

Benzene is notably more stable than would be expected from a Kekulé structure with alternating single and double bonds (which structure also does not explain the symmetrical hexagon shape found for the molecule). Here six adjacent carbon atoms have p atomic orbitals on them and so to explain the stability and geometry of benzene it is theorized that the actual structure is somewhere between the two extreme Kekulé structures written.
Another way of illustrating the presence of this phenomenon is to use dotted lines between atoms to indicate 'partial' bonding. In the case of benzene a single hexagon enclosing a circle is often used.

Note that resonance is introduced to explain increased stability and different geometry to that expected from a single Kekulé structure. Certain structures with p atomic orbitals on three or more adjacent sigma-bonded atoms are not more stable than expected and so resonance is not invoked. An example of this is cyclobutadiene. See aromaticity

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