Topic 15: Organic Chemistry A2: Carbonyls, Carboxylic Acids and Chirality

The chiral centre removes an internal equivalence between the two spatial arrangements, so the molecule and its mirror image can be distinct rather than two drawings of the same object.

Mark the asymmetric carbon, verify that all four attached groups differ, draw one tetrahedral arrangement, then reflect it across an imagined mirror plane. Keep two groups in the page and exchange the wedge/dash positions of the other two to show the mirror arrangement.

A correct pair has identical connectivity and opposite spatial arrangement at the chiral centre. If rotation can make every group coincide, the drawings are the same enantiomer.

Direction and measured angle are compared using the same wavelength, concentration, path length and temperature. The sign of rotation distinguishes the two samples experimentally.

Each enantiomer rotates plane-polarised light by an equal angle in the opposite direction under the same conditions. Their effects cancel, so the mixture has no overall optical activity.

If $S_N1$ and $S_N2$ occur together, both enantiomers may form but not in equal amounts: the inversion product receives the $S_N2$ contribution as well as part of the $S_N1$ contribution.

Use the reactant and product data together. Retention of a single optically active product supports stereospecific attack; loss of net rotation with a chiral product supports formation of both enantiomers.

Choose the longest chain containing the carbonyl carbon, number to give the carbonyl the lowest valid locant, then place other substituent prefixes alphabetically. In structural, displayed and skeletal formulae, preserve which carbon is double-bonded to oxygen.

When a higher-priority carbonyl suffix is used, an -OH group is named hydroxy: HOCH2COCH(CH3)CH3 is 1-hydroxy-3-methylbutan-2-one.

As the non-polar hydrocarbon chain grows, its contribution becomes larger and solubility in water falls even though the carbonyl oxygen can still accept hydrogen bonds.

Lithium tetrahydridoaluminate(III), LiAlH4, in dry ether reduces an aldehyde to a primary alcohol and a ketone to a secondary alcohol. Equations may use [H]. Water must be absent because the reagent reacts with it.

In nucleophilic addition, C=O is polarised Cδ+–Oδ-. A curly arrow starts at the lone pair on carbon of CN- and ends at the carbonyl carbon; a second arrow moves the C=O π pair to O. The O- intermediate then gains H from HCN, with the H-C bond pair returning to CN-. Attack on either face of a planar carbonyl can form a racemate.

For 2,4-DNPH identification, filter the derivative precipitate, recrystallise it to remove impurities, dry it, measure its melting temperature and compare with reference derivatives. The equation is not required.

In skeletal formulae, show the terminal C(=O)OH explicitly; the carbonyl carbon is part of the parent skeleton. Include E/Z notation and the double-bond locant when an unsaturated acid requires it.

A carboxylic acid can form more hydrogen-bond interactions with water than its ester isomer because the acid has an O-H donor as well as acceptor oxygens.

The nitrile carbon becomes the carboxyl carbon, so hydrolysis preserves the total number of carbon atoms in RCN.

Track the carbon skeleton while replacing or transforming only the carboxyl group. In esterification, the alcohol supplies the alkyl group attached to oxygen.

For an ester, locate the single oxygen between two carbon groups. The group bonded directly to that oxygen is the alkyl name; the carbonyl-containing side supplies the alkanoate name. Preserve C(=O)-O connectivity in structural, displayed and skeletal formulae.

To infer reactants, split the ester at the acyl C-O bond: methyl butanoate corresponds to butanoic acid (or butanoyl chloride) and methanol.

The carbonyl carbon is electron-poor and Cl is a good leaving group, so nucleophilic addition is followed by elimination. These reactions are rapid at room temperature and are not the reversible, acid-catalysed esterification of a carboxylic acid.

Hydrolysis or alcoholysis releases steamy acidic HCl fumes; reaction with ammonia or an amine can also form a white ammonium salt.

Hydrolyse every ester link. A triester needs three equivalents of hydroxide and forms the polyalcohol plus three carboxylate ions; a polyester yields monomer-derived products along its chain.

Terylene forms from benzene-1,4-dicarboxylic acid and ethane-1,2-diol. To draw a repeat section, retain both monomer carbon skeletons, show the -COO- links in the correct orientation and add continuation bonds through the chain.

To recover likely monomers from a repeat unit, cut each acyl C-O bond and restore -COOH/-OH (or -COCl/-OH) ends.

Using H = 1.0079, C = 12.0000 and O = 15.9949: propane, C3H8, has accurate $M_r=44.0632$, whereas CO2 has $M_r=43.9898$. A high-resolution peak near 44 can therefore distinguish them.

Generate formulae consistent with the accurate mass and any other evidence, then use fragment-ion formulae and m/z values to distinguish structural isomers when fragmentation data are supplied. Include the positive charge on a fragment ion.

Carbons related by molecular symmetry and with the same surroundings are equivalent and give the same signal; carbons in different surroundings give separate signals.

Propan-2-ol has two carbon environments: its two methyl carbons are equivalent by symmetry and the central C-OH carbon is different, so its carbon-13 spectrum has two peaks.

Label every carbon in a candidate structure by environment, group symmetry-related labels, predict the peak count, then match each environment to a permitted chemical-shift range. A carbonyl-region peak can support an ester, acid, aldehyde or ketone only when the precise range and other evidence agree.

An ethyl group often produces a three-proton triplet and a two-proton quartet: the CH3 protons have two neighbouring CH2 protons, while the CH2 protons have three neighbouring CH3 protons.

First count environments, then reduce integrated areas to the simplest whole-number ratio. Assign plausible shift ranges, use splitting to connect neighbouring fragments, and check that the assembled structure matches the molecular formula and every signal.

Low-resolution spectra separate environments but do not resolve the fine splitting. High resolution exposes multiplicity and therefore connectivity information.

Because different substances interact with the phases to different extents, they move at different average rates and become separated into spots or peaks.

If a spot moves 52 mm and $R_f=0.62$, the solvent front moved $52/0.62=84$ mm. Both distances must be measured from the same baseline in the same direction.

Under fixed paper/TLC, solvent and temperature conditions, a larger Rf means the substance travelled a greater fraction of the solvent-front distance. Values lie from 0 to 1.

Retention time is the time from injection until a component reaches the detector. Stronger retention by the stationary phase generally produces a longer retention time; the chromatogram x-axis is time.

GC-MS or HPLC-MS first separates a mixture, then mass spectrometry supplies mass/fragment evidence for each emerging peak. This supports sensitive analysis in forensics or drug testing in sport.