Chapter 19.SE, Problem 69AP

Propose structures for molecules that meet the following descriptions. Assume that the kinds of carbons (1°, 2°, 3°, or 4°) have been assigned by DEPT-NMR.

a) C₆H₁₂O;IR: 1715 cm^-1; ¹³C NMR: 8.0 δ (1°), 18.5 δ (1°), 33.5 δ (2°), 40.6 δ (3°), 214.0 δ (4°)

b) C₅H₁₀O; IR: 1730 cm^-1; ¹³C NMR: 22.6δ (1°), 23.6 8 (3°), 52.8 8 (2°), 202.4 8 (3°)

c) C₆H₈O; IR: 1680 cm^-1; ¹³C NMR: 22.9 δ (2°), 25.8 8 (2°), 38.2 δ (2°). 129.8 8 (3°), 150.6 8 (3°), 198.7 δ (4°)

Interpretation Introduction

a)

Interpretation:

A structure for the molecule which meets the descriptions given is to be assigned.

C₆H₁₂O; IR: 1715 cm^-1; ¹³CNMR: 8.0 δ (1⁰), 18.5 δ (1⁰), 33.5 δ (2⁰), 40.6 δ (³⁰), 214.0 δ (4⁰)

Concept introduction:

Aldehydes and ketones show a strong absorption band in IR from 1660-1770 cm^-1. Aldehydes show two characteristic C-H absorptions between 2700-2760cm^-1 and 2800-2860 cm^-1. Saturated aldehydes absorb near 1730 cm^-1 while aromatic aldehydes and α, β- unsaturated aldehydes absorb near 1705 cm^-1. Saturated ketones and cyclohexanones absorb near 1715 cm^-1 while aromatic ketones and α, β- unsaturated ketones absorb near 1685-1690 cm^-1. Cyclopentanones absorb around 1750 cm^-1.

In ¹³CNMR, carbons in double bond which are sp² hybridized absorb from 110 to 220 δ. Saturated aldehydes and ketones usually absorb in the region from 200 to 215 δ While aromatic and unsaturated carbonyl compounds absorb in the 190 to 200 δ region. The primary alkyl carbon absorbs in the range 10-15 δ, a secondary alkyl radical in the range 16-25 δ while a tertiary alkyl in the range 25-35 δ. The signals will be observed at downfield if these alkyl groups are attached to electron withdrawing groups.

To assign:

A structure for the molecule which meets the descriptions given.

C₆H₁₂O; IR: 1715 cm^-1; ¹³CNMR: 8.0 δ (1⁰), 18.5 δ (1⁰), 33.5 δ (2⁰), 40.6 δ (³⁰), 214.0 δ (4⁰)

Answer:

Answer to Problem 69AP

A structure for the molecule which meets the spectral descriptions given is

Explanation of Solution

The molecular formula of the compound is C₆H₁₂O. Ignoring oxygen the molecular formula of the parent compound becomes C₆H₁₄. Thus the compound has two hydrogen atoms (14-12) less than the parent and hence has one unsaturation unit perhaps a C=O.

The IR absorption at 1715 cm^-1 and ¹³CNMR peak at 214.0 δ indicate that the compound is a ketone. The ¹³CNMR further indicates that the carbonyl group is flanked by a secondary and tertiary carbon indicated by the two downfield signals at 33.5 δ and 40.6 δ. The remaining three carbons has to be primary, of which one is attached as methyl(8.0 δ) to the secondary carbon and two others are attached as two methyl groups (18.5δ) on the tertiary carbon. Hence the compound is 2-methyl-3-pentanone.

Conclusion

A structure for the molecule which meets the spectral descriptions given is

Interpretation Introduction

b)

Interpretation:

A structure for the molecule which meets the descriptions given is to be assigned.

C₅H₁₀O; IR: 1730 cm^-1; ¹³CNMR: 22.6 δ (1⁰), 23.6 δ (3⁰), 52.8 δ (2⁰), 202.4 δ (3⁰).

Concept introduction:

Aldehydes and ketones show a strong absorption band in IR from 1660-1770 cm^-1. Aldehydes show two characteristic C-H absorptions between 2700-2760cm^-1 and 2800-2860 cm^-1. Saturated aldehydes absorb near 1730 cm^-1 while aromatic aldehydes and α, β- unsaturated aldehydes absorb near 1705 cm^-1. Saturated ketones and cyclohexanones absorb near 1715 cm^-1 while aromatic ketones and α, β- unsaturated ketones absorb near 1685-1690 cm^-1. Cyclopentanones absorb around 1750 cm^-1.

In ¹³CNMR, carbons in double bond which are sp² hybridized absorb from 110 to 220 δ. Saturated aldehydes and ketones usually absorb in the region from 200 to 215 δ While aromatic and unsaturated carbonyl compounds absorb in the 190 to 200 δ region. The primary alkyl carbon absorbs in the range 10-15 δ, a secondary alkyl radical in the range 16-25 δ while a tertiary alkyl in the range 25-35 δ. The signals will be observed at downfield if these alkyl groups are attached to electron withdrawing groups.

To assign:

A structure for the molecule which meets the descriptions given.

C₅H₁₀O; IR: 1730 cm^-1; ¹³CNMR: 22.6 δ (1⁰), 23.6 δ (3⁰), 52.8 δ (2⁰), 202.4 δ (3⁰).

Answer to Problem 69AP

A structure for the molecule which meets the spectral descriptions given is

Explanation of Solution

The molecular formula of the compound is C₆H₁₂O. Ignoring oxygen the molecular formula of the parent compound becomes C₆H₁₄. Thus the compound has two hydrogen atoms less than the parent and hence has one unsaturation unit, a carbonyl group.

The IR absorption at 1730 cm^-1 and ¹³CNMR peak at 202.4 δ indicate that the compound is a saturated aldehyde. The ¹³CNMR further indicates that the carbonyl group is attached to a secondary carbon indicated by the downfield signal at 52.8 δ. Of the remaining three carbons one has to be tertiary(23.6 δ) which is attached to two methyl groups(22.6 δ). Hence the compound is 2-methylbutanal.

Conclusion

A structure for the molecule which meets the spectral descriptions given is

Interpretation Introduction

c)

Interpretation:

A structure for the molecule which meets the descriptions given is to be assigned.

C₆H₈O; IR: 1680 cm^-1; ¹³CNMR: 22.9 δ (2⁰), 25.8 δ (2⁰), 38.2 δ (2⁰), 129.8 δ (3⁰) 150.6 δ (3⁰), 198.7 δ (4⁰).

Concept introduction:

Aldehydes and ketones show a strong absorption band in IR from 1660-1770 cm^-1. Aldehydes show two characteristic C-H absorptions between 2700-2760cm^-1 and 2800-2860 cm^-1. Saturated aldehydes absorb near 1730 cm^-1 while aromatic aldehydes and α, β- unsaturated aldehydes absorb near 1705 cm^-1. Saturated ketones and cyclohexanones absorb near 1715 cm^-1 while aromatic ketones and α, β- unsaturated ketones absorb near 1685-1690 cm^-1. Cyclopentanones absorb around 1750 cm^-1.

In ¹³CNMR, carbons in double bond which are sp² hybridized absorb from 110 to 220 δ. Saturated aldehydes and ketones usually absorb in the region from 200 to 215 δ While aromatic and unsaturated carbonyl compounds absorb in the 190 to 200 δ region. The primary alkyl carbon absorbs in the range 10-15 δ, a secondary alkyl radical in the range 16-25 δ while a tertiary alkyl in the range 25-35 δ.

To assign:

A structure for the molecule which meets the descriptions given.

C₆H₈O; IR: 1680 cm^-1; ¹³CNMR: 22.9 δ (2⁰), 25.8 δ (2⁰), 38.2 δ (2⁰), 129.8 δ (3⁰) 150.6 δ (3⁰), 198.7 δ (4⁰).

Answer to Problem 69AP

A structure for the molecule which meets the spectral descriptions given is

Explanation of Solution

The molecular formula of the compound is C₆H₈O. Ignoring oxygen the molecular formula of the parent compound becomes C₆H₁₄. Thus the compound has six hydrogen atoms (14-8= 6) less than the parent and hence has three unsaturation units.

The IR absorption at 1680 cm^-1 and ¹³CNMR peak at 198.7 δ indicate that the compound is an α, β- unsaturatedketone. The compound has three unsaturated units. α, β- unsaturatedketone accounts for two unsaturation units. Hence the compound contains a six membered ring. The signals at 129.8 δ (30) 150.6 δ (30) can be attributed to the carbons in double bond. The other three signals at 22.9 δ (20), 25.8 δ (20), 38.2 δ (20) can be attributed to three methylene groups. The signal at 38.2 δ (20) can be assigned to methylene carbon attached to carbonyl carbon, that at 25.8 δ (20) to the methylene carbon carbon attached to C4 while that at 22.9 δ (20) is due to methylene carbon C5.

Conclusion

A structure for the molecule which meets the spectral descriptions given is