Chemistry is the science of matter, especially its chemical reactions, but also its composition, structure and properties. Chemistry is concerned with atoms and their interactions with other atoms, and particularly with the properties of chemical bonds....
has two meanings, one in acid-base chemistry and one in structural organic chemistry.
Meaning in acid-base chemistry
Most commonly, the term refers to the enhancement of the acidity
An acid dissociation constant, Ka, is a quantitative measure of the strength of an acid in solution. It is the equilibrium constant for a chemical reaction known as dissociation in the context of acid-base reactions...
of an acid by itself. The effect is mainly observed at high concentrations. IUPAC recommends the term "homoassociation", but this term has not gained wide usage."
The ionization of an acid varies nonlinearly with concentration. This effect arises from the stabilization of the conjugate base by its formation of a hydrogen bond
A hydrogen bond is the attractive interaction of a hydrogen atom with an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. The hydrogen must be covalently bonded to another electronegative atom to create the bond...
to the parent acid. A well known case is hydrofluoric acid
Hydrofluoric acid is a solution of hydrogen fluoride in water. It is a valued source of fluorine and is the precursor to numerous pharmaceuticals such as fluoxetine and diverse materials such as PTFE ....
, which is a significantly stronger acid when concentrated than when dilute due to the following equilibria:
- 2 HF → H+ + HF2-
The bifluoride anion (HF2-
) encourages the ionization of HF by stabilizing the F-
. Thus, the usual ionization constant for hydrofluoric acid (10-3.15
) understates the acidity of concentrated solutions of HF. Carboxylic acid
Carboxylic acids are organic acids characterized by the presence of at least one carboxyl group. The general formula of a carboxylic acid is R-COOH, where R is some monovalent functional group...
s and phenol
Phenol, also known as carbolic acid, phenic acid, is an organic compound with the chemical formula C6H5OH. It is a white crystalline solid. The molecule consists of a phenyl , bonded to a hydroxyl group. It is produced on a large scale as a precursor to many materials and useful compounds...
s also exhibit this effect.
The effect of homoconjugation is often high in nonaqueous solutions, wherein dissociation is often low.
Meaning in structural organic chemistry
Homoconjugation also refers to orbital overlap
Orbital overlap was an idea first introduced by Linus Pauling to explain the molecular bond angles observed through experimentation and is the basis for the concept of orbital hybridisation. s orbitals are spherical and have no directionality while p orbitals are oriented 90° to one another...
of two π-systems separated by a non-conjugating group, such as CH2
.. For example, the molecule CH2
(1,4-pentadiene) is homoconjugated because the two C=C double bond
A double bond in chemistry is a chemical bond between two chemical elements involving four bonding electrons instead of the usual two. The most common double bond, that between two carbon atoms, can be found in alkenes. Many types of double bonds between two different elements exist, for example in...
s (which are π-systems because each double bond contains one π bond) are separated by of one CH2
group. With this meaning, the name refers to the fact that the molecule is a homolog
In chemistry, homology refers to the appearance of homologues. A homologue is a compound belonging to a series of compounds differing from each other by a repeating unit, such as a methylene group, a peptide residue, etcetera....
of a conjugated system
In chemistry, a conjugated system is a system of connected p-orbitals with delocalized electrons in compounds with alternating single and multiple bonds, which in general may lower the overall energy of the molecule and increase stability. Lone pairs, radicals or carbenium ions may be part of the...