What is the relationship between the members of the following pairs of structures? Are they structural or geometrical isomers or resonance contributors?
(a) Compounds having the same molecular formula but with different structures are called structural isomers. The given compounds have the same molecular formula but they differ in the position of the functional group (ketone group). In Structure I, the ketone group is at the C-3 of the parent chain (hexane chain) and in Structure II, the ketone group is at the C-2 of the parent chain (hexane chain). Hence, the given pair represents structural isomers.
(b) Compounds having the same molecular formula, the same constitution, and the sequence of covalent bonds, but with different relative positions of their atoms in space are called geometrical isomers. In structures I and II, the relative positions of Deuterium (D) and hydrogen (H) in space are different. Hence, the given pairs represent geometrical isomers.
(c) The given structures are canonical structures or contributing structures. They are hypothetical and individually do not represent any real molecule. Hence, the given pair represents resonance structures, called resonance isomers.
Organic Chemistry – Some Basic Principles and Techniques - Reaction Mechanism
SN1 Reaction Mechanism:
SN1 reaction mechanism takes place by following three steps –
Formation of carbocation
Attack of nucleophile
Deprotonation
SN2 Reaction Mechanism:
The SN2 reaction mechanism involves the nucleophilic substitution reaction of the leaving group (which generally consists of halide groups or other electron-withdrawing groups) with a nucleophile in a given organic compound.
are isomeric compounds. 
are functional group isomers.