Question

Draw the C4H802 isomer with IR 2934, 1730 (s), 1126 (s) cm-1

Answer 1

Answer:

• First step: unsaturation degrees.

The molecular formula allows the determination of unsaturation degrees:

The molecuela has 1 unsaturation degree, this means that the compound could have a ring a double bond C=C or a C=O bond.

• Second step: IR data.

The IR data show three signals:

• 2934 cm^-1 = typically C-H strecth absorption for C sp³
  • This indicates that there is not C=C bond, because the C-H strecth of C sp² appears above 3000 cm^-1
• 1730 cm^-1 = typically C=O strecth absorption
  • This indicates that unsaturation degree found is due to a C=O bond
  • The C=O absorption at 1730 cm^-1 is characteristically absorption for esters
    • ketones / carboxylic acids = 1710 cm^-1;
    • aldehydes = 1725 cm^-1 (also show two C-H absorption of CHO group at 2700 and 2800 cm^-1)
• 1126 cm^-1 = typically C-O strecth absorption

• Third step: draw the isomer.

With the molecular formula and the IR data, we can define the unknown compound as an ester with a molecular formula: C₄H₈O₂

There are only two possible structures:

With NMR data we can distinguish between these two isomers