Chemistry A2 Level Ocr F324 Flashcards

Front 1

1. Benzene

Back 1

• Benzene, C6H6, consists of a sigma-bonded framework of carbon and hydrogen atoms.
• Above and below the plane of atoms is a bond, which consists of a delocalised electron cloud.
• The Kekulé structure of benzene assumes that all the bonds are localised i.e. cannot move. However, evidence to support the delocalised form of benzene comes from bond lengths, enthalpy change of hydrogenation and resistance to reaction.
• Electrophiles (accept lone pair of electrons) can attack a benzene ring – usually resulting in an electrophilic substitution process as a mechanism for a reaction.

Front 2

2. Reactions of Benzene

Back 2

• Benzene’s chemical reactions tend to have a high activation energy due to the delocalised -electron system.
• Benzene can react with the following:
a) concentrated nitric acid in the presence of concentrated sulfuric acid to form nitrobenzene
b) a halogen in the presence of a halogen carrier to form a mono-halogenated benzene compound e.g. chlorobenzene or bromobenzene. This reaction is slower than with cyclohexene due to the enhanced thermodynamic stability provided by the delocalised  electron system in benzene.

Front 3

3. Phenols

Back 3

• Phenol has the molecular formula C6H5OH. It is used in the production of plastics, antiseptics, disinfectants and resins for paints.
• Phenol reacts with:
a) alkalis to form sodium phenoxide (a salt) and water
b) sodium to form sodium phenoxide and hydrogen.
• Phenol reacts rapidly with bromine to form 2,4,6-tribromophenol and hydrogen bromide. This reaction is faster than with benzene because the oxygen lone pair of electrons in phenol overlaps with the  system on the ring, increasing its electron density and facilitating electrophilic attack.

Front 4

4. Carbonyl Compounds

Back 4

• Carbonyl compounds include aldehydes and ketones.
• Aldehydes can be oxidised in the presence of acidified potassium dichromate(VI) to form carboxylic acids. The orange dichromate(VI) ion is reduced to the green chromium(III) ion.
• Ketones cannot be oxidised under these conditions.
• Aldehydes can be reduced using sodium borohydride to form primary alcohols.
• Ketones can be reduced using sodium borohydride to form secondary alcohols.

Front 5

5. Tests for Carbonyl Compounds

Back 5

• 2,4-dinitrophenylhydrazine (2,4-DNP/2,4-DNPH/ Brady’s reagent) is used to detect the presence of the >C=O group in aldehydes and ketones – if present, an orange precipitate is formed.
• The precipitate may be recrystallised in ethanol and its melting point measured – this information may then be used to determine the identity of the original molecule.
• Tollens’ reagent (ammoniacal silver(I) nitrate) is used to detect the presence of an aldehyde group. A silver mirror is formed on warming the aldehyde with Tollens’ reagent and a carboxylic acid is also formed. There is no reaction with ketones.

Front 6

6. Carboxylic Acid and Esters

Back 6

• Carboxylic acids contain the –COOH group. This group is highly polar, and that is why carboxylic acids:
a) have higher melting and boiling points than expected – hydrogen bonding between molecules
b) are water-soluble – hydrogen bonding between carboxylic acid and water molecules.
• Carboxylic acids are weak acids and will react with reactive metals, metal oxides (and hydroxides) and metal carbonates to form carboxylate salts containing the –COO– ion.

Front 7

7. Reactions of Carboxylic Acids

Back 7

• Carboxylic acids react reversibly with alcohols, in the presence of an acid catalyst, to form esters – used in perfumes and flavourings.
• Esters may be hydrolysed in the presence of:
a) hot alkalis to form the corresponding alcohol and the carboxylate salt
b) hot acids to form the corresponding alcohol and the carboxylic acid.
• Fats and oils are naturally occurring esters. The fatty acid part of the molecule may be either:
a) saturated or
b) unsaturated – can be either cis or trans.

Front 8

8. Amines

Back 8

• Amines are molecules containing a nitrogen atom –e.g. the primary amine functional group –NH2.
• Amines are bases since they can accept a proton by using the lone pair of electrons on the nitrogen atom.
• Amines react with acids to form salts.
• Aliphatic amines may be prepared by the substitution of halogenoalkanes with excess ammonia.
• Aromatic amines can be prepared by reducing nitroarenes with tin and concentrated hydrochloric acid.
• Azo dyes are made by reacting an aromatic amine with nitrous acid and then phenol in alkali.

Front 9

9. Amino Acids

Back 9

• The general formula for an -amino acid is RCH(NH2)COOH.
• As an amino acid has both an acidic group (–COOH) and a basic group (–NH2), it can act as both an acid (proton donor) and a base (proton acceptor).
• At a certain pH known as the isoelectric point, a zwitterion forms. This contains the –NH3+ group and the –COO– group in the same molecule.
• Amino acids polymerise to form proteins while proteins can undergo hydrolysis to form -amino acids.

Front 10

10. Optical Isomerism

Back 10

• Optical isomers are non-superimposible mirror images about an organic chiral centre – four different groups attached to a carbon atom.
• When a chiral molecule is synthesised in the laboratory, usually many optical isomers can form.
• However, if an enzyme is involved in the synthesis, normally only one optical isomer forms.
• If pharmaceutical products contain only one optical isomer the possibility of side effects is reduced and the pharmacological activity is improved.
• Separating optical isomers is expensive, so chiral synthesis producing just one isomer is better.

Front 11

11. Condensation Polymers

Back 11

• Examples of condensation polymers are polyesters (e.g. Terylene) and polyamides (e.g. nylon-6,6).
• When a condensation polymer is formed, a small molecule such as water or hydrogen chloride is also formed.
• Terylene is made from benzene-1,4-dicarboxylic acid and ethane-1,2-diol.
• Nylon-6,6 is made from 1,6-diaminohexane and hexane-1,6-dicarboxylic acid.
• Kevlar is a special type of nylon that is made from benzene-1,4-diamine and benzene-1,4-dicarboxylic acid.

Front 12

12. Hydrolysis and Degradable Polymers

Back 12

• Condensation polymers have chemical groups that are vulnerable to chemical attack from either acids or alkalis – polyesters (ester group) and polyamides (amide group). This process is known as hydrolysis and results in the breakdown of the polymer.
• Disposing of polymers is an environmental problem. Scientists are working to develop degradable polymers similar in structure to poly(lactic acid).
• Condensation polymers may photodegrade as the C=O bond absorbs radiation.

Front 13

12. Synthetic Routes

Back 13

• Novel, useful molecules can be synthesised using organic chemistry.
• A chiral molecule is more difficult to synthesise since many other optical isomers may also form – costly in money and time to separate or resolve the isomers.
• Enzymes, bacteria, chiral catalysts and chiral-starting points (e.g. L-amino acids) are used to promote the formation of one chiral product.
• The chemical reactions you study during your A level chemistry course may be used to suggest how a target molecule can be synthesised, starting with a particular organic molecule.

Front 14

14. Types of Chromatography

Back 14

• Chromatography is an analytical technique that separates components in a mixture between a mobile phase and a stationary phase.
• The mobile phase may be a liquid or a gas.
• The solid phase may be:
a) a solid (as in thin-layer chromatography(TLC)) or
b) a liquid or a solid on a solid support (as in gas chromatography (GC)).
• The Rf value measures the ratio of the distance moved by the solute to the distance moved by the solvent.
• GC does have limitations – e.g. similar compounds have similar retention times.

Front 15

15. Combining Mass Spectrometry with Chromatograpgy

Back 15

• Mass spectrometry (MS) can be combined with chromatography:
a) to provide a far more powerful analytical tool than using chromatography alone
b) to generate mass spectra that can be analysed or compared with a spectral database by a computer for positive identification of a component.
• GC–MS can be used in analysis (e.g. in forensics), environmental analysis, airport security and space probes.

Front 16

16. Proton NMR Spectroscopy

Back 16

• In NMR, protons (hydrogen atoms) in a sample absorb and emit low-energy radiowave radiation in the presence of a powerful magnetic field.
• The number of peaks gives information about the number of proton environments.
• The area under each peak gives information about the number of hydrogen atoms in each environment.
• Their horizontal position in a spectrum gives the chemical nature of each proton region.
• Protons that are adjacent to unequivalent protons may be split into more peaks, where n+1 gives the number of splits (if adjacent to n hydrogen atoms).

Front 17

17. Carbon-13 NMR Spectroscopy

Back 17

• The number of peaks in a carbon-13 spectrum is the same as the number of different carbon environments.
• The relative position of each peak on the horizontal axis (the chemical shift) suggests the chemical environment of a particular carbon atom.
• This means it is possible from the carbon-13 spectrum alone to suggest the likely structure of a molecule – by counting the number of peaks and deducing the chemical nature of the bonded atoms to each carbon