4.4.4 · D5Nitrogen-Containing Compounds
Question bank — Cyanides and isocyanides
The single question behind every item below is: which atom of the C≡N unit is bonded to R, carbon or nitrogen? Once you know that, smell, hydrolysis, reduction, and bonding all follow.
First, the shorthand this page uses
True or false — justify
Both cyanide and isocyanide contain a carbon–nitrogen triple bond.
True. The atoms and the triple bond are identical; only the connectivity to R differs — R on carbon = cyanide, R on nitrogen = isocyanide.
In an isocyanide the terminal carbon has a complete octet with no formal charge.
False. To keep the triple bond and an octet the carbon must carry a lone pair, making it divalent with a formal charge of (from FC ); nitrogen carries .
KCN and AgCN, since both contain the ion, give the same organic product with R–X.
False. In KCN the ion is nearly free so the softer carbon attacks (→ cyanide); in AgCN the Ag–C bond cages the carbon so only nitrogen's lone pair is free (→ isocyanide). See HSAB principle.
Hydrolysis of a nitrile and an isocyanide both simply add water and release nothing else.
False. A nitrile gives ; an isocyanide gives . Different bonds survive, so different fragments leave.
Reducing either or needs the same number of hydrogens.
True. Both add 4 H (2 H₂ equivalents) across the triple bond, but the product amine degree differs because of connectivity.
Reduction of every C≡N compound gives a primary amine because there is only one nitrogen.
False. Degree depends on how many R groups sit on N. A nitrile's N ends with one chain (1°); an isocyanide's N already carries R and gains a (2°).
The pleasant/mild smell belongs to the isocyanide.
False. Isocyanides are notoriously foul; nitriles are mild. This foul smell is the whole basis of the carbylamine test.
is called methanenitrile in IUPAC.
False. The nitrile carbon is counted, so with two carbons it is ethanenitrile (acetonitrile). Only the common name "methyl cyanide" names the alkyl part alone.
Spot the error
"."
Wrong product. AgCN blocks the carbon, so nitrogen attacks and the product is the isocyanide , not the cyanide.
"Nitrile hydrolysis: ."
These are the isocyanide products. A nitrile keeps its R–C bond, so R stays on the carboxyl carbon: . See Carboxylic acids.
"Carbylamine test works on any amine (1°, 2°, 3°)."
Only primary amines react with + alc. KOH to make the foul isocyanide; 2° and 3° amines lack the two N–H hydrogens the reaction consumes. See Amines preparation and properties.
"Reduction of gives (ethylamine)."
Error. R stays on N and the external carbon becomes a on the same N, giving (dimethylamine, a 2° amine), not a 1° amine with a new C–C bond.
"Formal charge on isocyanide nitrogen is ."
Sign flipped. Plugging into gives for nitrogen; the terminal carbon is the centre.
"You make a nitrile from an amide by adding water."
Backwards. You remove water: . Adding water would do the reverse (hydrolysis). See Amides and dehydration.
" attacking through carbon in an makes an isocyanide."
No. Attack through carbon forms the R–C bond → cyanide. Attack through nitrogen forms R–N → isocyanide. See Alkyl halides and SN2 substitution.
Why questions
Why does the same ion give different products depending on its counter-cation?
Because is ambident (two donor atoms, C and N). Which atom is available to attack — carbon in free KCN, nitrogen in covalent AgCN — decides the product, not the ion's identity.
Why is HSAB relevant to the KCN vs AgCN choice?
The soft carbon end of prefers the soft carbon of R–X, so free (KCN) attacks via carbon. When Ag ties up that carbon, the harder nitrogen must act instead. See HSAB principle.
Why does nitrile hydrolysis release ammonia while isocyanide hydrolysis releases formic acid?
Trace the surviving bond (red bond in the atom-tracing figure). In the nitrile R–C stays intact, so N departs as . In the isocyanide R–N stays intact, so the lone external carbon picks up oxygens and leaves as .
Why is the isocyanide carbon described as "carbene-like"?
It is divalent and carries a lone pair with a charge — an electron-rich, reactive centre resembling a carbene, which is why isocyanides are so reactive and smelly.
Why does the nitrile carbon count in IUPAC naming but the alkyl carbon count in the common name?
IUPAC names the whole longest chain including the C of C≡N (suffix -nitrile); the common "alkyl cyanide" name treats CN as a substituent and names only the alkyl group.
Why do both reductions add exactly 4 hydrogens?
A triple bond reduced to a saturated linkage (C–N single bond region) needs two H₂ molecules; whether the product is 1° or 2° amine, the H-count added is the same.
Edge cases
What happens to hydrogen cyanide, , where "R" is just H — is it still a nitrile?
Structurally yes (H on carbon), but it is the parent inorganic acid, so it is named hydrogen cyanide rather than an alkanenitrile; the connectivity logic still applies.
If R–X is a tertiary halide, will KCN cleanly give the nitrile by ?
No — bulky 3° halides resist backside attack and tend to eliminate instead, so the clean nitrile route works best for 1°/2° substrates. See Alkyl halides and SN2 substitution.
Is the terminal carbon in a cyanide also negatively charged like in an isocyanide?
No. In both atoms are neutral with full octets; the charge separation is unique to the isocyanide's connectivity.
Could you get an amine directly (no new C) by reducing a nitrile?
No. The nitrile's triple-bond carbon becomes a , so reduction always adds a carbon between R and N: . To lose that carbon you would need a different reaction.
Does the carbylamine test give a positive smell with a secondary amine that happens to have an N–H?
No. Even with one N–H, a 2° amine cannot form an isocyanide by this route; the foul-smell positive is diagnostic for primary amines only.
Recall One-line master key
Which atom holds R? ::: Carbon → cyanide (mild, → carboxylic acid + , → 1° amine, from KCN). Nitrogen → isocyanide (foul, → amine + , → 2° amine, from AgCN).