(2) Polar additions to alkynes:

Like alkenes, alkynes undergo electrophilic addition of reagents such as HX and X2 to the multiple bond. In the case of the triple bond the addition takes place in two stages.


Note that both HX additions follow the Markovnikov rule:

Terminal alkynes also undergo Markovnikov hydration in the presence of a mercury(II) salt as catalyst - but the final product is not an alcohol:

Internal alkynes are hydrated under acidic conditions without the need for a catalyst and give mixtures of products:

(3) Reductions of alkynes: (a) Hydrogen and a catalyst:

(b) Alkali metal in ammonia:


THE CARBON-NITROGEN TRIPLE BOND - THE CHEMISTRY OF NITRILES

Text references: McMurry (5th Edition) Chapters 21, p. 846 and pp. 873 - 878.

Nomenclature - the systematic rules for naming nitriles:

(1) Simple nitriles are named by adding the termination nitrile to the name of the longest carbon chain in the molecule including the nitrile carbon (numbered as carbon 1) in the count:

(2) More complex nitriles are named from the corresponding carboxylic acid:

The Preparation of Nitriles:

(1) SN2 displacement of halide by cyanide anion from primary alkyl halides:

(2) Dehydration of (i.e. removal of water from) primary amides:

Notes: (1) A primary amide is an amide unsubstituted on the amide nitrogen - amides with alkyl or other substituents on N cannot be dehydrated. (2) SOCl2 is thionyl chloride and POCl3 is phosphorus oxychloride. Both are powerful dehydrating agents.

Reactivity of Nitriles:

The reactions of nitriles are dominated by nucleophilic attack at the electron-poor nitrile carbon:

(1) Hydrolysis (i.e. reaction with water) to amides and/or carboxylic acids:

Nitrile hydrolysis can be carried out under either acidic or basic conditions:

Subsequent hydrolysis of amide (RCONH2) to the corresponding carboxylic acid (RCO2H) is generally rapid under acidic conditions. (These - and similar - reactions of carbonyl compounds will be studied in Module CM2006).

Under mild basic conditions the hydrolysis stops at the amide stage:

(2) Reduction by hydride anion reagents to amines:

The reagent lithium aluminium hydride (LiAlH4, LAH) behaves chemically as a source of hydride anion, H–, a strong reducing agent:

(3) Reaction with organomagnesium compounds, RMgX, (Grignard Reagents) to yield ketones.

Grignard reagents, RMgX, act as a source of nucleophilic carbanions, R– :

Simplified mechanism:

Organomagnesium comounds and other organometallic reagents will be dealt with in more detail in P2 Module CM2005.