67E - Organic Chemistry (Mcmurry)

Question

The two ¹H NMR spectra shown here belong to crotonic acid (trans-${\mathbf{CH}}_{\mathbf{3}}\mathbf{CH}\mathbf{=}{\mathbf{CHCO}}_{\mathbf{2}}\mathbf{H}$) and methacrylic acid (${\mathbf{CH}}_{\mathbf{2}}\mathbf{=}\mathbf{C}\left({\mathrm{CH}}_3\right){\mathbf{CO}}_{\mathbf{2}}\mathbf{H}$). Which spectrum corresponds to which acid? Explain.

Step-by-Step Solution

Answer

The spectrum (a) is of Crotonicacid and the spectrum (b) is Methacrylic acid.

Step 1: NMR spectrum

NMR stands for nuclear magnetic resonance spectrum, which helps to determine the molecular structure and chemical composition of organic compound.

Step 2: Structures of acids:

Step 3: Explanation of a) 1 HNMR spectrum

The spectrum a) shows a methyl peakat 1.91, which can be identified easily as split into a doublet, because it is attached to a carbon atom with one hydrogen.The peaks at 7.10and 5.83 shows trans hydrogen atoms. Hence spectrum represents Crotonic acid.

Step 4: Explanation of b) 1 H NMR spectrum

The spectrum a) shows a methyl peak at 1.93, which can be identified easily as it is singlet because it is attached to a carbon atom withouta hydrogen atom. The peaks at 5.66 and 6.25 show hydrogen atoms. Hence spectrum b) represents Methacrylic acid.

Result:

NMR spectrum a) represents Crotonic acid and b) represents Methacrylic acid.