Chaper 9 : Alkynes

Chapter 9 : Alkynes

Alkynes

Nomenclature:
Functional group suffix = -yne (review)
Disubstituted alkynes, R-CºC-R', are described as "internal" alkynes
Monosubstituted alkynes, R-CºC-H, are described as "terminal" alkynes.


ethyne (or acetylene)			propyne
terminal			terminal


1-butyne			2-butyne
terminal			internal

Classify each of the following as an internal or a terminal alkyne:
(a) 1-hexyne	(c) cyclooctyne
(b) 3-octyne	(d) propyne

Stability:

As with alkenes, the more highly substituted internal alkynes are more stable.
By comparing thermodynamic data of alkynes and alkenes, it can be seen that the "extra" p bond in an alkyne is weaker than the alkene p bond:

290 kJ/mol

69.2 kcal/mol

126 kJ/mol

30.2 kcal/mol

So CºC to C=C = 164 kJ/mol (39 kcal/mol) while C=C to C-C = 126 kJ/mol (30.2 kcal/mol)
Therefore the "extra" p bond is 38 kJ/mol (8.8 kcal/mol) weaker that an alkene p bond.

Rank 1-hexyne and 3-hexyne for each of the following properties:
(a) heat of hydrogenation		(c) heat of formation
(b) heat of combustion		(d) stability

Structure:

The alkyne functional group consists of two sp hybridised C atoms bonded to each other via one s and two p bonds.
The 2 p bonds are produced by the side-to-side overlap of the two pairs of p-orbitals not utilised in the hybrids.
The substituents are attached to the CºC via s bonds.
The 2 C of the CºC and the 2 atoms attached directly to the CºC are linear.
Since alkynes are linear, they cannot exist as cis- / trans- isomers.


The two separate perpendicular p molecular orbitals			Combined p molecular orbitals

Physical Properties:

As with hydrocarbons in general, alkynes are non-polar and are insoluble in water but soluble in non-polar organic solvents.

Reactivity:

The p bonds are a region of high electron density (red) so alkynes are typically nucleophiles.
Alkynes typically undergo addition reactions in which one or both of the p-bonds are converted to new s bonds.
Terminal alkynes, R-CºC-H, are quite acidic (indicated by blue) for hydrocarbons, pKa = 26
Deprotonation of a terminal acetylene gives an acetylide ion.
The acetylide ion is a good nucleophile and can be alkylated to give higher alkynes.