Chemistry - Spectroscopy

A brief overview of NCEA level 3 chemistry, spectroscopy topic.

What does IR spectroscopy tell us?

Functional groups present in a molecule.

What does 13C NMR spectroscopy tell us?

Number and type of carbon environments and structural information.

What does mass spectrometry tell us?

Molecular mass and fragment ions.

Why do we use IR, NMR and MS together?

Each provides different information that combines to identify the unknown molecule.

What functional group characterises alkanes?

C-C single bonds only.

What suffix do alkanes use?

-ane

What functional group characterises alkenes?

C=C double bond.

What suffix do alkenes use?

-(position)-ene

What functional group characterises alkynes?

C≡C triple bond.

What suffix do alkynes use?

-(position)-yne

What functional group characterises alcohols?

-OH

What suffix do alcohols use?

-an-(position)-ol

What functional group characterises amines?

-NH2

What suffix do amines use?

-an-(position)-amine

What functional group characterises carboxylic acids?

-COOH

What suffix do carboxylic acids use?

-anoic acid

What functional group characterises ketones?

C=O within the carbon chain.

What suffix do ketones use?

-an-(position)-one

What functional group characterises aldehydes?

-CHO

What suffix do aldehydes use?

-al

What functional group characterises acid chlorides?

-COCl

What suffix do acid chlorides use?

-oyl chloride

What functional group characterises amides?

-CONH2

What suffix do amides use?

-anamide

What functional group characterises esters?

O=C-O-C

How are esters named?

Alkyl alkanoate (right side first, then left side)

What happens when IR radiation passes through a molecule?

Bonds vibrate by stretching or bending.

When does a bond absorb IR radiation?

When the vibration frequency matches the IR frequency.

What causes a peak in an IR spectrum?

Absorption of IR radiation by a bond vibration.

What does a dip in an IR spectrum represent?

Absorption of IR radiation.

What are the axes on an IR spectrum?

x-axis = wavenumber (cm-1), y-axis = % transmittance.

What is the functional group region?

1500–4000 cm-1.

What is the fingerprint region?

Below 1500 cm-1.

What IR peak indicates an alcohol?

Broad O-H peak at 3200–3550 cm-1.

What additional peak supports an alcohol?

Strong C-O peak around 970–1250 cm-1.

What IR peak indicates a carboxylic acid?

Very broad O-H peak from 2500–3300 cm-1.

What second peak confirms a carboxylic acid?

Strong C=O peak at 1705–1720 cm-1.

What IR peak indicates a primary amine?

Two N-H peaks around 3300–3500 cm-1.

What IR peak indicates an aldehyde?

C=O peak around 1720–1740 cm-1.

What confirms an aldehyde?

Two small peaks at 2690–2840 cm-1.

What IR peak indicates a ketone?

Strong C=O peak around 1710–1720 cm-1.

How do you distinguish a ketone from an aldehyde?

Ketones lack aldehyde peaks at 2690–2840 cm-1.

What IR peak indicates an ester?

C=O peak around 1735–1750 cm-1.

What confirms an ester?

Strong C-O peaks between 1000–1300 cm-1.

What IR peaks indicate an alkene?

C=C at 1630–1680 cm-1 and =C-H above 3000 cm-1.

Which carbon isotope is used in NMR?

Carbon-13.

Why is 13C used instead of 12C?

13C has nuclear spin

12C does not.

What is electron shielding?

Electrons reduce the magnetic field experienced by the nucleus.

What causes deshielding?

Electronegative atoms pulling electron density away.

What happens to deshielded carbons?

They appear further downfield (higher ppm).

What does the number of peaks represent in 13C NMR?

Number of carbon environments.

Does peak height matter in 13C NMR?

No.

If two carbons are in identical environments, how many peaks do they produce?

One peak.

What does a peak at 205–220 ppm suggest?

Ketone carbonyl carbon.

What does a peak at 190–200 ppm suggest?

Aldehyde carbonyl carbon.

What does a peak at 160–185 ppm suggest?

Carboxylic acid/ester/amide/acid chloride carbonyl.

What does a peak at 115–140 ppm suggest?

Alkene carbon.

What does a peak at 50–70 ppm suggest?

Carbon bonded to oxygen in an alcohol.

What is the molecular ion?

M+, formed when a molecule loses an electron.

What does the molecular ion peak tell us?

Molecular mass.

What is the base peak?

The tallest peak on the spectrum.

What does the base peak represent?

Most abundant ion fragment.

If the molecular ion mass is odd, what is likely present?

One nitrogen atom.

If the molecular ion mass is even, what is likely present?

No nitrogen (only C, H, O)

What fragment corresponds to m/z = 15?

CH3+

What fragment corresponds to m/z = 17?

OH+

What fragment corresponds to m/z = 28?

CO+

What fragment corresponds to m/z = 29?

CH3CH2+ or CHO+

What fragment corresponds to m/z = 31?

CH3O+ or CH2OH+

What fragment corresponds to m/z = 43?

C3H7+ or CH3CO+

What fragment corresponds to m/z = 45?

COOH+

What fragment corresponds to m/z = 57?

C4H9