Rubber composition and pneumatic tire

1. A rubber composition comprising:

a diene rubber;

silica; and

a heterocyclic compound (where, the heterocyclic compound does not have a silicon atom) which has a hydrocarbon group having from 8 to 30 carbons and at least one type of heterocycle selected from the group consisting of a piperazine ring, a morpholine ring, and a thiomorpholine ring,

a content of the silica being from 50 to 200 parts by mass per 100 parts by mass of the diene rubber, and

a content of the heterocyclic compound being from 0.5 to 20% by mass with respect to the content of the silica,

wherein the heterocyclic compound is represented by Formula (I):

in Formula (I), X 7 represents a nitrogen atom, an oxygen atom or a sulfur atom,

X 3 , X 4 , X 5 , and X 6 each independently represent a hydrogen atom or a hydrocarbon group,

when X 7 is a nitrogen atom, n3 is 1, and one or both of X 1 and X 2 each independently represent Formula (I-1):-(A 1 ) n1-1 —R 1-1 ,

when only one of X 1 and X 2 represents Formula (I-1), the remaining groups represent at least one type selected from the group consisting of a hydrogen atom, a sulfone-based protecting group, a carbamate-based protecting group, and Formula (I-3):—(R 2 —O) n2 —H,

in Formula (I-3), R 2 each independently represents a divalent hydrocarbon group,

n2 represents from 1 to 10,

when X 7 is an oxygen atom or a sulfur atom, n3 represents 0, and X 1 represents Formula (I-1):-(A 1 ) n1-1 —R 1-1 ,

in Formula (I-1), A 1 represents a carbonyl group and/or Formula (I-2): —R 1-2 (OH)—O—,

n1-1 represents 0 or 1,

R 1-1 represents the hydrocarbon group having from 8 to 30 carbons, and

in Formula (I-2), R 1-2 represents a trivalent hydrocarbon group.

2. The rubber composition according to claim 1 , wherein the heterocyclic compound is a compound represented by Formula (I), where X 7 is a nitrogen atom, and n3 is 1, and

both of X 1 and X 2 each independently represent Formula (I-1).

3. The rubber composition according to claim 1 , wherein the heterocyclic compound is a compound represented by Formula (I), where X 7 is a nitrogen atom, and n3 is 1,

only one of X 1 and X 2 represents Formula (I-1),

the remaining groups represent at least one type selected from the group consisting of a hydrogen atom, a sulfone-based protecting group, a carbamate-based protecting group, and Formula (I-3):—(R 2 —O) 2 —H, In Formula (I-3), R 2 each independently represents a divalent hydrocarbon group, and n2 represents from 1 to 10.

4. The rubber composition according to claim 1 , wherein the heterocyclic compound is a compound represented by Formula (I), where X 7 is an oxygen atom or a sulfur atom, n3 is 0, and

X 1 represents Formula (I-1).

5. A pneumatic tire comprising the rubber composition described in claim 1 .