Alkynes Flashcards
alkynes
compounds containing a C=-C triple bond
- sp hybridized - p orbitals make pie bonds - sp orbital makes sigma bond
alkynes are similar to alkenes in their ability to function as
either a base or acid
Nomenclature of Alkynes
same naming rules apply
- use yne at the end
if a double and a triple bond are in equivalent positions then you list
double bond first but still name the final part with yne
a lower pKa value is
higher in acidity
acetylide ion
conjugate base of acetylene
- has a negative charge
the equilibrium of an acid-base reaction favors the formation of
the weaker acid and weaker base
greater stability leads to a weaker base and stronger acid
- an sp orbital will hold electrons closer to + leading to weaker base or stronger acid
- sp3 will hold electron density farther away leading to a stronger base and weaker acid
OH- cannot
deprotonate acetylene
terminal alkynes are acidic and
can be deprotonated
alkynide ion
the conjugate base of a terminal alkyne
bases to deprotonate a terminal alkyne
- H2N-
- H-
- C4H9-
- H-C=-C-
bases that cannot deprotonate a terminal alkyne
- T-buO-
- EtO-
- OH-
alkynes can be prepared from alkyl dihalides
- required two successive elimination(E2) reactions
- two E2 reactions because the alkyl halide will have two leaving groups
germinal
a dihalide with both halogens connected to the same carbon atom
vicinal
a dihalide with the two halogens being connected to adjacent carbon atoms
both germinal and vicinal dihalides require two successive elimination reactions
- the second elimination reaction must be a very strong base
- sodium amide(NaNH2) dissolved in liquid ammonia(NH3)
using NaNH2 dissolved in NH3 is frequently used for the preparation of terminal alkynes
the strongly basic conditions favor production of an alkynide ion which serves as the driving force for the overall process
three equivalents of the amide ion are required to prepare an alkyne
- 2 for the success E2 reactions
- 1 to deprotonate the terminal alkyne and form the alkynide ion
after the alkynide ion has formed and the reaction is complete, a proton source can
be introduced into the reaction vessel thereby protonating the alkynide ion to regenerate the terminal alkyne
a terminal alkyne can be prepared by
treating a dihalide with excess(xs) sodium amide(NaNH2/NH3) followed by water(H2O)
poisoned catalyst
a partially deactivated catalyst, a poisoned catalyst, it is possible to convert an alkyne into a cis alkene(without further reduction)
lindlar’s catalyst
a common poisoned catalyst used to convert an alkyne into a cis alkene(without further reduction)
using a poisoned catalyst does not produce
any trans alkene
syn addition is used to form the
cis alkene as the major products
explain the difference between NaNH2 and Na,NH3
- NH2- is a very strong base
- Na and NH3 are a source of electrons for reagents to use
dissolving metal reduction
- alkynes can be reduced to trans by dissolving metal reduction
- Na/NH3(l)
radical anion
an intermediate with an unpaired electron instead of a pair of electrons
to produce an alkane an alkyne can be treated with
H2 in the presence of a metal catalyst, such a Pt, Pd, or Ni
to produce a cis alkene an alkyne can be treated with
H2 in the presence of a poisoned catalyst such as Lindlars catalyst of Ni2B
to produce a trans alkene an alkyne can be treated with
sodium(Na) in liquid ammonia(NH3 l)
Hydrohalogenation of Alkynes
- follows same rules as alkenes
- markinokov addition
- rate = k[alkyne][HX]^2
- if H2O2 is used then the X will go to the least substituted position(like alkenes)
