Organichemica
Basic Organic Chemistry

A Tutorial

Dave Woodcock ©2001

OUC Home Page Chem Home Page
Level 1

Glossary

H

Main Index
Glossary Index


Hammond Postulate

Although there is no direct connection between the energy of a compound and the energy of a transition state in a reaction involving that compound, Hammond posulated that in certain circumstances a parallel could be drawn between the two energies. In particular, if an intermediate with an energy similar to the energy of the transition state is formed during a reaction, then a parallel between the energy of the transition state and of the intermediate is possible. Thus any feature of the intermediate which either reduces or increases its energy will have a proportional effect on the energy of the transition state (reducing or increasing it respectively).
Because the intermediate and the transition state must be of similar energy for this postulate to apply, we are looking here at high-energy intermedates such as carbocations or radicals.
If the high energy intermediate is stabilized by some effect, then the transition state will be proportionally stabilized, the activation energy will be lowered and the rate of formation of the intermediate will be increased. Thus the postulate that most stable intermediate will be formed the fastest and will dominate the reaction.

Heisenberg Uncertainty Principle

The position and velocity (energy, momentum) of particles such as the electron cannot both be known simultaneously with certainty. As a consequence the electron in an atom cannot be described mathematically using Newtonian mechanics (which requires that position and velocity be known simultaneously) and must instead be described in terms of a wave using quantum mechanics.

homolytic/heterolytic cleavage

Theoretically, when a two electron bond is broken there are two possibilities for the movement of the two bonding electrons:

Homolytic cleavage means that the two electrons go one to each of the two atoms originally bonded. The species formed in this process are radicals.

Heterolytic cleavage means that both electrons go to one of the atoms (and none to the other). The species formed in this process are ions.

hydrolysis

A term indicating that water is both the solvent and a reactant.

hydrogen bond
A strong attraction between:
a hydrogen atom polarized positive by attachment to a small electronegative atom and
a small electronegative atom.
Because the electronegative atom must be small, hydrogen bonds involve F, O, and N as the small electronegative atom.
The strongest H-bonds involve the more electronegative F atom. H-bonds with O are next, and the weakest involve N.

Glossary Index
A B C D E F G H I J K L M
N O P Q R S T U V W X Y Z

Main Index

page upkeep: Dave.
I would appreciate comments and additions by email to:
woodcock@okanagan.bc.ca
This page last modified on: