Ch 9 : Alkynes

Chapter 9 : Alkynes

Alkynes

Nomenclature:
Functional group suffix = -yne (review)
Disubstituted alkynes, R-C≡C-R', are described as "internal" alkynes because the C≡C unit is "inside" the structure.
Monosubstituted alkynes, R-C≡C-H, and the unsubstituted alkyne (ethyne) H-C≡C-H are described as "terminal" alkynes because the C≡C unit at the end of the structure.


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 ANSWER	(b) 3-octyne ANSWER
(c) cyclooctyne ANSWER	(d) propyne ANSWER

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" π bond in an alkyne is weaker than the alkene π bond:

H_h 1-hexyne =

290 kJ/mol

(

69.2 kcal/mol

) vs 1-hexene =

126 kJ/mol

(

30.2 kcal/mol

)

So C≡C 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" π bond is 38 kJ/mol (8.8 kcal/mol) weaker that an alkene π bond.

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

Structure:

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


The two separate perpendicular π molecular orbitals			Combined π 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 π 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 π-bonds are converted to new σ bonds.
Terminal alkynes, R-C≡C-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.