yeast Mothers and brewers have been familiar with
yeast for centuries. It is considered to be the oldest
“plant” cultivated by humans. Yeast is a unicellular fun-
gus that belongs to the family Saccharomycetaceae. It
lives in the soil, on plants, and in the air, and it has been
used in the production of bread, beer, and wine because
it is responsible for the process of fermentation. It pro-
duces carbon dioxide and alcohol when in the presence
of sugar. There are actually many species of yeasts.
yield The quantity of product obtained from a chemi-
cal reaction.
ylide A CHEMICAL SPECIES produced (actually or
notationally) by loss of a HYDRON from an atom
directly attached to the central atom of an ONIUM ION,
e.g.,Yukawa-Tsuno equation A multiparameter exten-
sion of the HAMMETT EQUATIONto quantify the role
of enhanced RESONANCEeffects on the reactivity of
meta- and para-substituted benzene derivatives, for
example,
lg k= lg ko+ ρ[σ+ r(σ+– σ)]
The parameter r gives the enhanced resonance effect on
the scale (σ+–σ) or (σ––σ), respectively.
See alsoRHO(ρ) VALUE; SIGMA(σ) CONSTANT.Ph P –CHRR 3 + ′→(Ph P –C RR 3 +–′←→ylide Ph P=CR 3 R)′′285