hydrophobic interaction The tendency of hydrocar-
bons (or of lipophilic hydrocarbonlike groups in
solutes) to form intermolecular aggregates in an aque-
ous medium, and analogous intramolecular interac-
tions. The name arises from the attribution of the
phenomenon to the apparent repulsion between water
and hydrocarbons. Use of the misleading alternative
term hydrophobic bondis discouraged.
hydrophobicity The association of nonpolar groups
or molecules in an aqueous environment that arises
from the tendency of water to exclude nonpolar
molecules.
See alsoLIPOPHILICITY.
hydroxyl group A functional group that has a
hydrogen atom joined to an oxygen atom by a polar
covalent bond (–OH).
hydroxyl ion One atom each of oxygen and hydro-
gen bonded into an ion (OH–) that carries a negative
charge.
See alsoION.
hydroxyl radical A radical consisting of one hydro-
gen atom and one oxygen atom. It normally does not
exist in a stable form.
hyperconjugation In the formalism that separates
bonds into σ- and π-types, hyperconjugation is the
interaction of σ-bonds (e.g. C–H, C–C, etc.) with a π-
network. This interaction is customarily illustrated by
CONTRIBUTING STRUCTUREs, e.g., for toluene (below),
sometimes said to be an example of heterovalent or
sacrificial hyperconjugation, so named because the con-
tributing structure contains one two-electron bond less
than the normal LEWIS FORMULAfor toluene:
At present, there is no evidence for sacrificial
hyperconjugation in neutral hydrocarbons. The con-
cept of hyperconjugation is also applied to CARBENIUM
IONs and RADICALs, where the interaction is now
between σ-bonds and an unfilled or partially filled πor
p-ORBITAL. A contributing structure illustrating this for
the tert-butyl cation is:This latter example is sometimes called an example
of isovalent hyperconjugation (the contributing struc-
ture containing the same number of two-electron bonds
as the normal Lewis formula).
Both structures on the right-hand side are also
examples of double-bond no-bond resonance.
The interaction between filled πor porbitals and
adjacent antibonding σ* orbitals is referred to as nega-
tive hyperconjugation, as for example in the fluo-
roethyl anion:See alsoPI(π) BOND; SIGMA(σ) BOND; SIGMA PI; N-σ
DELOCALIZATION.hyperfine SeeELECTRON PARAMAGNETIC RESONANCE
SPECTROSCOPY.hyperpolarization An electrical state where the
inside of a cell is made more negative relative to the
outside than was the case at resting potential of about
–70mV.hypertonic solution A solution whose solute con-
centration is high enough to cause water to move out
of cells via osmosis.134 hydrophobic interaction