OCR Organic Chemistry Revision

Flashcards covering organic analysis, NMR interpretation, chromatography, and functional group tests based on the OCR exam preparation materials.

How does Gas Chromatography (GC) separate compounds in a mixture when the column is coated with a liquid polymer?

Separation occurs by the relative solubility of the compounds in the stationary phase (the liquid polymer).

What is the role of Gas Chromatography-Mass Spectrometry (GC-MS) in identifying products after separation?

The products are identified by comparison of their fragmentation patterns with a spectral database or library.

What chemical is used as a standard for chemical shift measurements in NMR spectroscopy, and at what chemical shift is it found?

Tetramethylsilane (TMS), which is found at $\delta = 0\,ppm$.

Why is $CDCl_3$ used as a solvent in $^1H$ NMR spectroscopy?

It contains no hydrogen atoms (protons), so it does not produce a signal that would interfere with the sample's spectrum.

What is the purpose of adding $D_2O$ to a sample before $^1H$ NMR analysis?

It is used to identify $O-H$ and $N-H$ protons through proton exchange, as these peaks disappear from the spectrum when $D_2O$ is added.

What is meant by the term 'isoelectric point' in the context of $\alpha$-amino acids?

The pH at which the amino acid exists as a zwitterion (a dipolar ion with no net charge).

Describe the chemical test results of Tollens' reagent when reacted with an aldehyde versus a ketone.

An aldehyde will form a silver mirror or black precipitate, whereas a ketone will show no observable change.

What observations would confirm the presence of a carbonyl group in an organic compound using 2,4-dinitrophenylhydrazine (Brady's reagent)?

The formation of an orange, yellow, or red precipitate.

How can the products of a 2,4-DNP test be used to distinguish between different carbonyl compounds without using spectroscopy?

By recrystallising the orange precipitate, measuring its melting point, and comparing the value to a known database or library of derivative melting points.

According to the transcript, what are two benefits of using single stereoisomers in drug synthesis?

Reducing side effects and increasing pharmacological activity or effectiveness.

What chemical test identifies an unsaturated carbon chain, and what is the positive observation?

Reacting with bromine or bromine water ($Br_{2(aq)}$), which results in the solution decolourising (going from orange to colourless).

In Thin-Layer Chromatography (TLC), how is the $R_f$ value calculated?

$$R_f = \frac{\text{distance moved by the spot}}{\text{distance moved by the solvent front}}$$

What reagent and conditions are needed for the reduction of an aldehyde to a primary alcohol, such as pentan-1-ol?

Sodium tetrahydridoborate(III) ($NaBH_4$) in aqueous solution.

In mass spectrometry, what does the molecular ion peak ($M^+$) represent?

The peak at the highest $m/z$ value (ignoring isotopes) which corresponds to the relative molecular mass ($M_r$) of the compound.

What is the systematic name for the ester compound identified as 'Ester A' with the structure $CH_2BrCH_2COOCH_2CH_3$?

Ethyl 3-bromopropanoate.

Why does phenol undergo nitration more readily than benzene?

The lone pair of electrons on the oxygen atom is partially delocalised into the $\pi$-system, increasing the electron density and making the ring more susceptible to electrophilic attack.

What happens to the structure of an amino acid like aspartic acid in a solution with high pH?

The carboxylic acid groups lose protons to form carboxylate groups ($-COO^-$), resulting in a negatively charged ion.

A compound with the formula $C_{10}H_{12}O_2$ shows a singlet at $\delta = 2.3\,ppm$ ($3H$), a doublet at $\delta = 1.6\,ppm$ ($3H$), and a quartet at $\delta = 2.7\,ppm$ ($1H$). What structure is suggested?

The compound is a carboxylic acid with an aromatic ring: $CH_3C_6H_4CH(CH_3)COOH$.

How can you distinguish between 2-nitrophenol, 3-nitrophenol, and 4-nitrophenol using $^{13}C$ NMR spectroscopy?

4-nitrophenol has symmetry and shows only 4 carbon environments (peaks), whereas 2-nitrophenol and 3-nitrophenol have no symmetry and show 6 carbon environments (peaks).

What reagent is used to distinguish between ethanol ($CH_3CH_2OH$) and phenol ($C_6H_5OH$)?

Bromine water; phenol will react to form a white precipitate and decolourise the bromine water, while ethanol will not react.