Home / Courses / Organic Chemistry / Unit 5

Organic Chemistry Unit 5: Alcohols, ethers and spectroscopy

Alcohols and ethers have characteristic reactions, and spectroscopy identifies unknown compounds from their spectra.

Covers: Alcohol reactions and oxidation, ethers and epoxides, IR spectroscopy, mass spectrometry, ¹H and ¹³C NMR spectroscopy

Study guide

Alcohols have an OH group and hydrogen bond, giving them high boiling points. They can be made from alkenes, by reducing carbonyls with NaBH₄ or LiAlH₄, or by Grignard reagents (RMgX) adding to carbonyls. To substitute an OH, chemists convert it into a good leaving group, using HX, SOCl₂, PBr₃ or tosyl chloride. Oxidation depends on the alcohol type: primary alcohols oxidize to aldehydes with PCC or to carboxylic acids with chromic acid (Jones reagent); secondary alcohols oxidize to ketones; tertiary alcohols don't oxidize. Ethers (R–O–R) are fairly unreactive and are made by the Williamson ether synthesis, an SN2 reaction of an alkoxide with a primary alkyl halide. Epoxides open under basic conditions at the less substituted carbon and under acidic conditions at the more substituted carbon.

Infrared (IR) spectroscopy identifies functional groups by bond vibrations. Key peaks: a broad O–H stretch at 3200–3600 cm⁻¹ for alcohols, a very broad O–H at 2500–3300 cm⁻¹ for carboxylic acids, N–H around 3300–3500 cm⁻¹, a strong C=O stretch near 1700 cm⁻¹, C≡N or C≡C near 2100–2250 cm⁻¹, and C–H stretches near 3000 cm⁻¹. Mass spectrometry (MS) measures mass-to-charge ratio; the molecular ion (M⁺) gives the molar mass, an M+2 peak about one-third the size of M signals chlorine, and an M+2 peak about equal to M signals bromine.

Nuclear magnetic resonance (NMR) spectroscopy maps carbon and hydrogen environments. In ¹H NMR, the number of signals shows distinct hydrogen environments, chemical shift (in ppm) shows the electronic environment (alkyl H near 1 ppm, H next to O around 3–4 ppm, alkene H 4.5–6.5, aromatic H 6.5–8, aldehyde H 9–10, carboxylic acid H 10–12), integration shows the relative number of hydrogens, and splitting follows the n + 1 rule (n neighboring hydrogens split a signal into n + 1 peaks). ¹³C NMR shows the number of unique carbons, with carbonyl carbons around 160–220 ppm.

Heating an alcohol with H₂SO₄ or H₃PO₄ removes water to form an alkene. Secondary and tertiary alcohols react by E1 through a carbocation, so rearrangements can happen and Zaitsev's rule predicts the major product. Tertiary alcohols react fastest. In mass spectrometry, molecules break at weak bonds into stable cations. Benzyl compounds give a strong peak at m/z 91 (the tropylium ion). Alcohols often lose water (M − 18), and carbonyl compounds and amines cleave at the bond next to the functional group (α-cleavage).

Typical ¹³C NMR shifts: sp³ alkyl carbons at 10–50 ppm, carbons bonded to O, N or a halogen at about 40–90 ppm, alkene and aromatic carbons at 100–150 ppm, nitrile carbons around 115–125 ppm, and carbonyl carbons at 160–220 ppm. Symmetry-equivalent carbons give a single peak. Chromium(VI) reagents (chromic acid/Jones reagent, PCC) oxidize alcohols and are themselves reduced to Cr(III). A carbon's oxidation level rises as it gains bonds to oxygen and loses bonds to hydrogen: alcohol, then aldehyde or ketone, then carboxylic acid.

Key ideas

• Primary alcohols oxidize to aldehydes or acids; secondary to ketones; tertiary don't oxidize.
• The Williamson ether synthesis is an SN2 reaction.
• IR: broad O–H around 3300 cm⁻¹ and strong C=O near 1700 cm⁻¹.
• MS molecular ion gives molar mass; M+2 peaks reveal Cl and Br.
• ¹H NMR: signals, chemical shift, integration and splitting (n + 1).

Formulas and rules

Common mistakes

• Thinking tertiary alcohols oxidize to ketones; they don't oxidize.
• Counting the hydrogens on the same carbon when applying the n + 1 rule.

Words to know

Alcohol

A compound with an OH group on an sp³ carbon.

Grignard reagent

An organomagnesium halide that acts as a strong nucleophile.

PCC

A mild oxidizer that turns primary alcohols into aldehydes.

Jones reagent

Chromic acid, which oxidizes primary alcohols to carboxylic acids.

Tosylate

A good leaving group made from an alcohol and tosyl chloride.

Ether

A compound with an oxygen bonded to two carbons.

Williamson ether synthesis

SN2 reaction of an alkoxide with an alkyl halide.

Epoxide opening

Ring-opening of an epoxide by a nucleophile.

IR spectroscopy

Identifies functional groups by bond vibrations.

Wavenumber

IR frequency unit, in cm⁻¹.

Mass spectrometry

Measures the mass-to-charge ratio of ions.

Molecular ion

The ion of the whole molecule, giving its molar mass.

NMR spectroscopy

Maps hydrogen and carbon environments using magnetic fields.

Chemical shift

The position of an NMR signal, in ppm.

Integration

The area under an NMR signal, showing relative H count.

Splitting

Division of an NMR signal by neighboring hydrogens.

n + 1 rule

n neighboring H's split a signal into n + 1 peaks.

Sample practice questions

5 more questions like these, from level 1 to 5, are in the practice set.