Question

Short answer

a.\({{\rm{C}}_{\rm{a}}} < {{\rm{C}}_{\rm{b}}} < {{\rm{C}}_{\rm{c}}}\)

b.\({{\rm{C}}_{\rm{b}}} < {{\rm{C}}_{\rm{c}}} < {{\rm{C}}_{\rm{a}}}\)

Step-by-step solution

Chemical shift

\(^{{\bf{13}}}{\bf{C}}\)spectroscopy gives a downfield (higher value) chemical shift and an upfield chemical shift according to the non-equivalent chemical environment present around the atom which is under observation.

Explanation

a.

\({{\rm{C}}_{\rm{c}}}\)is adjacent to the carbonyl oxygen which is an electronegative atom. Therefore,\({{\rm{C}}_{\rm{c}}}\)has higher value of chemical shift.\({{\rm{C}}_{\rm{b}}}\)is also close to carbonyl oxygen followed by\({{\rm{C}}_{\rm{a}}}\).

Therefore, the order of chemical shift value is shown as:

\({{\rm{C}}_{\rm{a}}} < {{\rm{C}}_{\rm{b}}} < {{\rm{C}}_{\rm{c}}}\)

b.

\({{\rm{C}}_{\rm{a}}}\)has higher value of chemical shift due to presence of\({\rm{s}}{{\rm{p}}^{\rm{2}}}\)hybridized atoms.\({{\rm{C}}_{\rm{c}}}\)is adjacent to Br which is an electronegative atom. Therefore,\({{\rm{C}}_{\rm{c}}}\)has higher value of chemical shift.\({{\rm{C}}_{\rm{b}}}\)is also close to the double bond.

Therefore, the order of chemical shift value is shown as:

\({{\rm{C}}_{\rm{b}}} < {{\rm{C}}_{\rm{c}}} < {{\rm{C}}_{\rm{a}}}\)