Edexcel A Level Chemistry: Analytical Techniques, Chirality, and Carbonyls

Vocabulary practice flashcards covering key definitions, analytical techniques, chirality, and carbonyl compound reactions from Topics 7, 17, and 19.

Molecular ion ($M$)

The peak in a mass spectrum with the highest $m/z$ value, which gives the relative molecular mass ($M_r$) of the compound.

Fragmentation (Mass Spectrometry)

The breakdown of a molecular ion inside a mass spectrometer into a detected positive ion ($X^+$) and an uncharged, invisible radical ($Y^\bullet$).

High-resolution mass spectrometry

An analytical technique measuring accurate masses to 4 decimal places to distinguish between different molecules that share the same whole-number relative molecular mass ($M_r$).

Fingerprint region

The region of an infrared spectrum below $1500\,\text{cm}^{-1}$ that is unique to a specific molecule and can be compared against a database for identification.

Tetramethylsilane (TMS)

An inert, volatile standard reference compound assigned a chemical shift of $\delta = 0\,\text{ppm}$ in NMR spectroscopy because all of its hydrogen and carbon atoms are equivalent.

$n+1$ rule

A rule in ${}^1\text{H}$ NMR spectroscopy stating that a peak splits into $n+1$ lines, where $n$ is the number of hydrogen atoms on adjacent carbon atoms.

Retention time

The time taken from sample injection to detection in gas chromatography (GC) or high-performance liquid chromatography (HPLC).

Chiral centre

A carbon atom bonded to four different atoms or groups, denoted by an asterisk (*), which gives rise to optical isomerism.

Enantiomers

Optical isomers that are non-superimposable mirror images and rotate the plane of plane-polarised monochromatic light by equal amounts in opposite directions.

Racemic mixture

A 50:50 mixture of two enantiomers that is optically inactive because the equal and opposite rotations of plane-polarised light cancel out.

$S_N1$ mechanism

A two-step nucleophilic substitution mechanism proceeding via a planar carbocation intermediate, allowing equal attack from either face to yield an optically inactive racemic mixture.

$S_N2$ mechanism

A single-step nucleophilic substitution mechanism where the nucleophile attacks from the opposite side of the halogen via a transition state, resulting in inversion of configuration while retaining optical activity.

Tollens' reagent

An aqueous reagent used to distinguish aldehydes from ketones; warming it with an aldehyde reduces silver ions to form a silver mirror, while ketones give no change.

Fehling's / Benedict's solution

An alkaline copper reagent that oxidises aldehydes, turning from a blue solution to a brick-red precipitate of $\text{Cu}_2\text{O}$, but remaining blue with ketones.

2,4-DNPH (Brady's reagent)

A qualitative chemical test for carbonyl compounds that forms an orange or yellow precipitate when mixed with aldehydes or ketones.

Iodoform test

A reaction using $\text{I}_2$ and $\text{NaOH}(aq)$ that produces a pale yellow precipitate of $\text{CHI}_3$ with an antiseptic smell, confirming the presence of a methyl ketone ($\text{CH}_3\text{CO}-$) group or ethanal.