Topic 15

Topic 15 on a page

Everything from this topic in one place, for the last read before an exam. If anything here is unfamiliar, follow the link back to the full page.

Values and data

Quantity	Value
accurate relative atomic masses	H 1.0078, C 12.0000, N 14.0031, O 15.9949
TMS	δ = 0 ppm, the reference for both carbon-13 and proton NMR
carbon-13 shifts / ppm	C–C 5–55, C–O 50–90, C=C 90–150, C=O in acids and esters 160–185, C=O in aldehydes and ketones 190–220
proton shifts / ppm	CH₃ at the end of a chain 0.7–1.2, CH₂ in a chain 1.2–1.4, next to C=O 2.0–2.7, bonded to O 3.3–4.3, –CHO 9.4–10.0, –COOH 10–12
boiling temperatures / °C	butane −1, propanal 48, propanone 56, propan-1-ol 97, ethanoic acid 118

The reaction web

A reaction web with eight boxes. Halogenoalkane to nitrile with KCN. Nitrile to carboxylic acid with dilute HCl under reflux. Primary alcohol to aldehyde with oxidation and distillation, and aldehyde back to alcohol with LiAlH4. Aldehyde to carboxylic acid by oxidation under reflux. Carboxylic acid to acyl chloride with PCl5. Acyl chloride to ester with an alcohol and to an amide with ammonia. — The main routes in this topic.

Change	Reagent and conditions	Page
aldehyde to carboxylic acid	acidified K₂Cr₂O₇, reflux, or Tollens' or Fehling's	Oxidation tests
aldehyde to primary alcohol; ketone to secondary alcohol	LiAlH₄ in dry ether, then water or acid	Reduction
aldehyde or ketone to hydroxynitrile	HCN with a little KCN, room temperature	HCN
primary alcohol to acid	acidified K₂Cr₂O₇, excess, reflux	Making acids
nitrile to acid	dilute HCl, reflux; or NaOH(aq), reflux then acidify	Making acids
halogenoalkane to nitrile	KCN in ethanol, reflux	Making acids
acid to primary alcohol	LiAlH₄ in dry ether	Reactions of acids
acid to salt	NaOH, carbonate or hydrogencarbonate	Reactions of acids
acid to acyl chloride	PCl₅ (dry), misty fumes of HCl	Reactions of acids
acid to ester	alcohol, a few drops of concentrated H₂SO₄, heat; equilibrium	Esterification
acyl chloride to acid, ester, amide, N-substituted amide	water, alcohol, concentrated NH₃, amine; room temperature	Acyl chlorides
ester to acid and alcohol	dilute acid, reflux (reversible)	Hydrolysis of esters
ester to salt and alcohol	NaOH(aq), reflux (complete)	Hydrolysis of esters
diacid + diol to polyester	condensation, water lost at each link	Polyesters

The tests

Test	Positive result	Shows
Tollens' reagent, warm	silver mirror	aldehyde
Fehling's or Benedict's, warm	blue to brick-red precipitate	aldehyde
acidified dichromate(VI), warm	orange to green	aldehyde (also primary and secondary alcohols)
2,4-DNPH	orange precipitate; melting temperature of the derivative names the compound	aldehyde or ketone
iodine and alkali, warm	pale yellow precipitate of CHI₃	CH₃CO– group (ethanal, methyl ketones)
sodium carbonate	effervescence of CO₂	carboxylic acid
PCl₅	misty fumes of HCl	–OH group (alcohol or acid)

Definitions to learn word for word

Spectroscopy in brief

Evidence	What it gives
accurate mass of the molecular ion	molecular formula: add the accurate atomic masses and match to four decimal places
number of carbon-13 peaks	number of carbon environments (equivalent carbons give one peak)
carbon-13 shift	type of carbon, using the ranges above
number of proton peaks	number of hydrogen environments
proton peak area	ratio of hydrogens in each environment
proton splitting	n neighbouring hydrogens give n + 1 lines: singlet 1, doublet 1 : 1, triplet 1 : 2 : 1, quartet 1 : 3 : 3 : 1
$R_\text{f}$	distance moved by spot ÷ distance moved by solvent front, between 0 and 1

Common mistakes

Chiral centre: "four different atoms". Say atoms or groups.
S_N2 and optical activity: S_N2 gives one enantiomer, which is optically active. It is S_N1 that gives a racemic (inactive) product.
Racemic mixtures contain chiral molecules but have no net rotation.
Aldehyde formula: write CHO, never COH. Ester name: the alcohol part comes first.
Carbonyl hydrogen bonds: aldehydes and ketones cannot hydrogen-bond to each other, but can accept a hydrogen bond from water.
Acid wanted from an alcohol: reflux with excess oxidant. Distil only to collect an aldehyde.
Alkaline hydrolysis gives the salt, not the acid. Esterification is an equilibrium, but alkaline hydrolysis and acyl chloride reactions go to completion.
Spectra: count peaks by environments, not atoms. The CH₂ of an ethyl group is a quartet because of the 3 hydrogens on the neighbouring CH₃.
$R_\text{f}$ : measure from the origin line to the centre of the spot, and use the same line for the solvent front.

Mixed test

Define a chiral centre and state whether butan-2-ol and propan-2-ol have one.
A pure enantiomer of a tertiary bromoalkane reacts with hydroxide ions and the product is optically inactive. Name the mechanism and explain.
Write the equation and give the conditions for the conversion of propanal into propan-1-ol, and state what type of alcohol is formed.
Name the two reagents and the conditions for converting bromoethane into propanoic acid. How many carbon atoms does the product have compared with the starting material?
Write an equation for the reaction of ethanoyl chloride with ethanol, and explain why the reaction goes to completion but esterification does not.
An organic compound has a molecular ion peak at 74.0366. Calculate which of C₃H₆O₂ (use H = 1.0078, C = 12.0000, O = 15.9949) and C₄H₁₀O (accurate mass 74.0729) it is.
Predict the number of peaks in the carbon-13 NMR spectrum of ethyl ethanoate, and the pattern of the proton NMR peaks.
A spot moves 3.6 cm when the solvent front moves 9.0 cm. Calculate its $R_\text{f}$ and state what would happen to the $R_\text{f}$ value if a different solvent were used.

Answers cover these until you have tried the questions

A carbon atom bonded to four different atoms or groups. Butan-2-ol has one (C2: H, OH, CH₃, CH₂CH₃). Propan-2-ol does not, as its central carbon has two CH₃ groups.
S_N1. The bromide ion leaves first to form a planar carbocation. The hydroxide ion attacks from either side with equal probability, giving equal amounts of the two enantiomers, a racemic mixture, which is optically inactive.
$\ce{CH3CH2CHO + 2[H] -> CH3CH2CH2OH}$ , using lithium tetrahydridoaluminate(III) in dry ether. A primary alcohol (propan-1-ol) is formed.
Step 1: KCN in ethanol, heated under reflux, to give propanenitrile. Step 2: dilute hydrochloric acid, heated under reflux. The product has 3 carbon atoms, one more than the 2 in bromoethane, because the carbon of the CN group becomes the carboxyl carbon.
$\ce{CH3COCl + CH3CH2OH -> CH3COOCH2CH3 + HCl}$ . The reaction goes to completion because the acyl chloride is very reactive and the HCl escapes as a gas, so it is not reversible. Esterification from an acid and an alcohol is reversible, and reaches an equilibrium that contains the reactants and products.
C₃H₆O₂: 3 × 12.0000 + 6 × 1.0078 + 2 × 15.9949 = 36.0000 + 6.0468 + 31.9898 = 74.0366, which matches the measured value. C₄H₁₀O is 74.0729 and does not. The compound has molecular formula C₃H₆O₂.
4 peaks in carbon-13 NMR (four carbon environments: C=O, OCH₂, CH₃CO and CH₃). Proton NMR has three peaks: a singlet (3 H, CH₃CO), a quartet (2 H, OCH₂) and a triplet (3 H, CH₃).
$R_\text{f}$ = 3.6 ÷ 9.0 = 0.40. A different solvent changes how soluble each substance is in the mobile phase, so the $R_\text{f}$ value would change.