Encyclopedia of Chemistry

where heat enters into a system; energy is absorbed by
the reactant.
See alsoEXOTHERMIC.

ene reaction The addition of a compound with a
double bond having an allylic hydrogen (the ene) to a
compound with a multiple bond (the enophile) with
transfer of the allylic hydrogen and a concomitant reor-
ganization of the bonding, as illustrated below for
propene (the ene) and ethene (the enophile). The
reverse is a “retro-ene” reaction.

energy Classically defined as the capacity for doing
work, energy can occur in many forms such as heat
(thermal), light, movement (mechanical), electrical,
chemical, sound, or radiation. The first law of thermo-
dynamics is often called the law of conservation of
energy and states that energy cannot be created or
destroyed, only transformed from one form into
another.

energy of activation(Arrhenius energy of activation;
activation energy Ea), (SI unit: kJ mol–1) An opera-
tionally defined quantity expressing the dependence of
a rate constant on temperature according to

Ea= RT^2 (lnk/T)p

as derived from the Arrhenius equation, k = A
exp(–Ea/RT), where A (SI unit: as for the corresponding
rate constant) is termed the “pre-exponential factor.”
See alsoENTHALPY OF ACTIVATION.

energy profile SeeGIBBS ENERGY DIAGRAM; POTEN-
TIAL-ENERGY PROFILE.

enforced concerted mechanism Variation of the

reaction parameters in a series of reactions proceeding

in nonconcerted steps may lead to a situation where the

putative intermediate will possess a lifetime shorter

than a bond vibration, so that the steps become con-

certed. The TRANSITION STATEstructure will lie on the

coordinate of the More O’Ferrall–Jencks diagram lead-

ing to that of the putative intermediate.

enhanced greenhouse effect The natural GREEN-

HOUSE EFFECThas been enhanced by anthropogenic

emissions of greenhouse gases. Increased concentra-

tions of carbon dioxide, methane, nitrous oxide, CFCs,

HFCs, PFCs, SF 6 , NF 3 , and other photochemically

important gases caused by human activities, such as

fossil fuel consumption and added waste to landfills,

trap more infrared radiation, thereby exerting a warm-

ing influence on the climate.

See alsoGLOBAL WARMING.

enophile SeeENE REACTION.

entatic state A state of an atom or group that, due to

its binding in a protein, has its geometric or electronic

condition adapted for function. Derived from entasis

(Greek), meaning tension.

entering group An atom or GROUP that forms a

BONDto what is considered to be the main part of the

SUBSTRATEduring a reaction, for example, the attack-

ing NUCLEOPHILEin a bimolecular nucleophilic SUBSTI-

TUTION REACTION.

enterobactin A SIDEROPHOREfound in enteric bacte-

ria such as Escherichia coli; sometimes called ente-

rochelin.

enterochelin SeeENTEROBACTIN.

enthalpy The property of a system that is equal to E

+ PV. Eis the internal energy of the system, Pis the

enthalpy 97