Compound Y is a colourless liquid with a sharp, unpleasant odour, used industrially as a chemical intermediate. In its mass spectrum, compound Y shows a molecular ion peak at m/z = 116 and a prominent fragment ion peak at m/z = 99. Its infrared spectrum shows a strong, very broa…

Answer: ((a)) Mr = 116 ((b)) Carboxylic acid: the very broad absorption spanning 2500–3000 cm^-1 is the characteristic hydrogen-bonded O–H absorption of a carboxylic acid, and the absorption at 1715 cm^-1 falls within the C=O range for an acid; the complete absence of any absorption at 3200–3600 cm^-1 rules… ((c)) Mass lost = 116 - 99 = 17, corresponding to loss of (hydroxyl radical), leaving the acylium fragment C5H11CO^+ at m/z = 99. ((d)) CH3CH2CH2CH2CH2COOH, hexanoic acid.. Part (a): Relative molecular mass from the molecular ion The molecular ion, M^+, is the whole molecule having lost just one electron, so its m/z value gives the relative molecular mass directly: Mr(Y) = 116 Part (b): Identifying the functi…

AS Chemistry 9701: Combining Mass Spectrometry and IR to Identify a Carboxylic Acid

Bond	Functional group	Wavenumber / cm⁻¹
O–H	carboxylic acid	2500–3000
O–H	alcohol	3200–3600
C=O	carboxylic acid	1700–1725
C=O	ester	1710–1750

Bond

Functional group

Wavenumber / cm⁻¹

O–H

carboxylic acid

2500–3000

O–H

alcohol

3200–3600

C=O

carboxylic acid

1700–1725

C=O

ester

1710–1750

Part (a): Relative molecular mass from the molecular ion

The molecular ion, $\text{M}^+$ , is the whole molecule having lost just one electron, so its $m/z$ value gives the relative molecular mass directly:

$M_r(\text{Y}) = 116$

Part (b): Identifying the functional group from the infrared spectrum

Two features of the spectrum are diagnostic:

The absorption at $1715\ \text{cm}^{-1}$ lies in the C=O range for a carboxylic acid ( $1700$ – $1725\ \text{cm}^{-1}$ ).
The absorption spanning $2500$ – $3000\ \text{cm}^{-1}$ is unusually broad, far broader than a normal C–H absorption, which is the classic signature of a hydrogen-bonded acid O–H bond (carboxylic acids exist largely as hydrogen-bonded dimers, which broadens and shifts this absorption to lower wavenumber than an isolated O–H).
There is no absorption in $3200$ – $3600\ \text{cm}^{-1}$ , which is where an alcohol’s O–H absorption would appear instead. Its absence rules out an alcohol as a possibility.

Together, the C=O absorption and the broad low-wavenumber O–H absorption identify compound Y as a carboxylic acid.

Part (c): Interpreting the fragment ion at m/z = 99

The neutral fragment lost has mass:

$116 - 99 = 17$

A mass of $17$ matches the hydroxyl group, $\text{OH}$ ( $16 + 1 = 17$ ), lost as the radical $\bullet\text{OH}$ . This leaves behind the acylium cation:

$\text{C}_5\text{H}_{11}\text{CO}^+ \quad \left(m/z = 5(12) + 11(1) + 12 + 16 = 99\right)$

Part (d): Deducing the structure of compound Y

Putting the evidence together:

$M_r = 116$ together with a carboxylic acid functional group (parts (a) and (b)) fits the molecular formula $\text{C}_6\text{H}_{12}\text{O}_2$ .
Loss of $\bullet\text{OH}$ (mass 17) to leave the $\text{C}_5\text{H}_{11}\text{CO}^+$ acylium ion (mass 99) shows the hydroxyl is bonded directly to a $-\text{C}_5\text{H}_{11}\text{CO}-$ carbon skeleton.
Given that the carbon skeleton is unbranched, this fixes the structure as a straight chain of six carbons with the acid group at one end.

This is consistent with:

$\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{COOH}$

Hexanoic acid.

Final answers

(a) $M_r = 116$
(b) Carboxylic acid, the broad O–H absorption (2500–3000 cm⁻¹) and the C=O absorption at 1715 cm⁻¹ together identify it; the absence of any absorption at 3200–3600 cm⁻¹ rules out an alcohol.
(c) Fragment lost $= 17$ ( $\bullet\text{OH}$ ); ion formed is $\text{C}_5\text{H}_{11}\text{CO}^+$ .
(d) $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{CH}_2\text{COOH}$ , hexanoic acid.