Question #1560d

1 Answer
Jun 2, 2015

Nucleophilic substitution by #"KCN"# gives an alkyl cyanide, while #"AgCN"# gives an alkyl isocyanide because #"KCN"# is ionic and #"AgCN"# is covalent.

Explanation:

Alkyl halides react with alcoholic #"KCN"# to form alkyl cyanides

#"KCN"# is an ionic compound. The nucleophile is the cyanide ion

#[:stackrel("-1") ("C")"≡N:"]^"-"#

The carbon atom is the nucleophile and the reaction is

#underbrace("RX")_color(red)("alkyl halide") + :stackrel("-")("C")"≡N:" stackrel("Δ"color(white)(m)) (→)underbrace("RCN")_color(red)("alkyl cyanide") + "X"^"-"#

Alkyl halides react with alcoholic silver cyanide to form alkyl isocyanides.

The electronegativity difference between #"Ag"# and #"C"# is so small that the #"Ag-C"# bond is over 90 % covalent.

The nucleophile is #"Ag-C≡N:"#.

The carbon atom is no longer available to act as a nucleophile, but the N atom can still attack the alkyl halide.

#underbrace("RX")_color(red)("alkyl halide") + "AgCN:" stackrel("Δ"color(white)(m)) (→)underbrace("RNC")_color(red)("alkyl isocyanide") + "AgX"#