OCR Chemistry A level

What is the empirical formula of Benzene?

CH

What is the state of benzene at room temperature?

Liquid

What is the bond angle of benzene?

Bond angle = 120°

What are the 3 features of benzene that don’t support Kekule’s model?

● Benzene is resistant to addition reactions

● Enthalpy change of hydrogenation of benzene is more stable than predicted

● All the carbon bonds are same length

What technique was used to find the bond lengths of benzene?

X ray diffraction

What happens to the 4th electron in the p-orbital of each carbon atom in benzene?

It delocalises to form rings of electron density above and below the hexagon, forming rings of delocalised electron density above/below the hexagon.

How do the rings of electron density affect the stability of Benzene?

Makes benzene very stable, even though it is unsaturated (aromatic stability).

Why does benzene have a relatively high melting point?

Close packing of flat hexagonal molecules when solid.

Is benzene soluble in water? Explain why?

No because it is non polar.

Dangers of benzene - why it is not used in schools?

It is a carcinogen.

How do you name compounds containing a benzene ring?

-benzene, or phenyl- ; can designate position on ring using numbers if there is more than one substituent.

Why is benzene attacked by electrophiles?

High electron density above/below ring due to delocalised electrons.

Nitration of benzene is what type of reaction?

Electrophilic substitution reaction.

What is the catalyst in nitration of benzene?

Sulfuric acid

What type of catalyst is used for a Friedel-Crafts reaction?

A halogen carrier (e.g. AlCl 3)

Why does benzene not react directly with halogens?

The aromatic ring is too stable.

How could you use a Friedel-Crafts mechanism to add a methyl group to a benzene ring?

Use a halogenoalkane and AlCl3 to create an electrophile that can attack benzene.

What reactions can you carry out to show the weak acidity of phenol?

A neutralisation reaction with NaOH occurs but absent when you react phenol with carbonates.

What is the relative ease of electrophilic substitution of phenol compared to benzene and why?

It is easier for electrophilic substitution to occur with phenol because oxygen lone pair of electrons from the -OH group are partially delocalised into ring therefore this increases the electron density of the ring thus electrophiles are more attracted phenol.

What is the directing effect of electron donating groups OH and NH2?

They direct group to the 2 and 4 position of the ring in electrophilic substitution of aromatic compounds.

What is the directing effect of electron withdrawing group NO2?

NO2 directs atoms to the 3 position of the ring in electrophilic substitution of aromatic compounds.

What is the carbonyl group?

C=O

What is the functional group and general formula for an aldehyde?

RCHO (C double bonded to O, single bond to H and R).

What is the functional group for a ketone?

RCOR’ (C double bonded to O).

How do you name aldehydes?

-al suffix (C=O is on the end of a chain).

How do you name ketones?

-one suffix (designate number for which carbon C=O is on).

What kind of intermolecular forces do molecules with the carbonyl group have? Why?

Permanent dipole-dipole due to the polar C=O bond (O is ∂-).

How soluble are they in water? What influences solubility?

Yes - form hydrogen bonds between water molecules and oxygen of C=O. As C chain length increases, solubility decreases.

Which bond in carbonyl compounds is usually involved in reactions? Why?

C=O, due to the polarity of the bond (large difference in electronegativity between C and O).

What is the strongest bond in carbonyl compounds?

C=O

Why is the addition of HCN important?

Increases the length of the carbon chain by one carbon atom allowing more useful molecules to be made.

Will the product of HCN added to a carbonyl compound have optical isomers? Why?

Yes they will. In the aldehyde/ketone, the carbonyl carbon is planar, so the :CN - can attack from either above or below, forming two enantiomers.

What is the name of the product when HCN is added to a carbonyl compound?

Hydroxynitriles (have OH and CN groups).

What is Fehling’s solution? What colour is it?

Copper complex ions, blue.

What happens when an aldehyde is added to Fehling’s solution?

Reduced to Cu + ions → colour change to brick red ppt.

What happens when a ketone is added to Fehling’s solution?

No visible change → stays blue

How do you test for a carbonyl compound?

Use Brady’s reagent - 2,4-dinitrophenylhydrazine.

If a carbonyl compound is present an orange precipitate is formed.

What is in Tollens’ reagent?

Silver complex ions in colourless solution.

What happens when an aldehyde is added to Tollen’s reagent?

Silver mirror forms as Ag+ reduced to Ag (s).

What happens when a ketone is added to Tollen’s reagent?

No visible change.

What is another oxidising agent for alcohols and aldehydes? What change in colour does this undergo?

Acidified potassium dichromate (VI) - H2SO4 and K2Cr2O7. Colour change from orange to green.

What is a reducing agent for aldehydes and ketones? What ions does this release in the solution?

NaBH 4 (sodium tetrahydridoborate (III)), releases an H - ion.

What is the functional group of a carboxylic acid?

-COOH (C=O and C-OH)

How do you name carboxylic acids?

-oic acid

Are carboxylic acids soluble in water? Why? What influences their solubility.

Yes. Acid group can form hydrogen bonds with water molecules.

What are the intermolecular forces in carboxylic acids?

Hydrogen bonds in solid state - very strong.

What are esters (what are they formed from)? Functional group, general formula?

Formed from carboxylic acids and alcohols.

RCOOR’ (C=O, C-O-C)

How do you name esters?

Start with the alcohol group that has replaced the hydrogen, then acid part e.g. propyl (from alcohol) ethanoate (from carboxylic acid).

What characteristic physical properties do esters have?

Volatile, pleasant fruity smells e.g. apple, pear drops.

What are some uses of esters?

Flavourings, perfumes (both for longer chains), solvents (short chains), plasticisers.

How could you distinguish carboxylic acids from other -OH containing compounds?

Add NaHCO3 , acids will produce sodium salt, water and carbon dioxide.

Write an equation for the reaction of ethanoic acid with NaOH.

CH3COOH + NaOH → H2O + CH3COO- Na+

Write an equation for the reaction of ethanoic acid with calcium oxide

2CH3COOH + CaO → (CH3COO)2 Ca + H2O

What catalyst is needed for the formation of esters from alcohols and carboxylic acids?

Concentrated strong acid. e.g. H2SO4

What catalyst is needed for the hydrolysis of esters?

Dilute strong acid. e.g. H2SO4

What is an alternative method of hydrolysis?

Base hydrolysis

What are the advantages of base hydrolysis?

Reaction goes to completion due to neutralisation by base - more product in the mixture than acid catalysed hydrolysis.

How do you name amines?

-amine or amino-

Why are amines so reactive?

The lone pair of electrons on the Nitrogen - due to polar N-H bond.

What shape are amines around the N? Bond angle?

What shape are amines around the N? Bond angle? Trigonal pyramidal, 107° due to lone pair on N.

What kind of intermolecular forces do they have? Why?

Hydrogen bonding due to polar N-H bond and lone pair of electrons on N atom.

Do amines have intermolecular forces which are stronger than or weaker than alcohols? Why?

Weaker, as N has a lower electronegativity than O → Weaker hydrogen bonding

How can/when do amines act as bases?

The lone pair on the nitrogen atom accepts a proton.

How can/when do amines act as nucleophiles?

When they bond with an electron-deficient C atom (donate lone pair from N).

In order to be the strongest base, what must a particular amine have (out of a set of amines)?

Greatest electron density around the N atom, making it a better electron pair donor (attracts protons more).

What effect do alkyl groups have (on electron density and base strength)?

Positive inductive effect - increase electron density around N → stronger base

How can primary amines then form 2°, 3° amines and 4° ammonium ions?

Multiple substitutions; primary amine is a nucleophile that attacks the original haloalkane etc.

What are the problems with this method?

Not efficient as low yield of 1° amine due to multiple substitution.

How would you maximise the yield of the primary amine?

Use excess ammonia.

What type of mechanism is the reaction of a haloalkane with a cyanide ion?

Nucleophilic substitution

What conditions does this reaction require? What is the product formed?

Ethanol as a solvent. A nitrile is formed.

How do you get from a nitrile to a primary amine? (name of reaction type and reagents/catalysts).

Reduction using Nickel / Hydrogen catalyst.

Why is this a purer method of synthesising amines?

Only the primary amine can be formed.

What conditions are needed to form nitrobenzene from benzene?

Concentrated H2SO4 and HNO3 to form the NO2+ ion for electrophilic attack.

How do you form an ammonium chloride salt from nitrobenzene? What conditions are needed?

Reduce the nitrile using Tin / HCl → forms an ammonium salt with Cl - ions Room temperature

Equation for the reaction of nitrobenzene → phenylamine.

C6H5NO2 + 6[H] → C6H5NH 2 + 2H2O

What mechanism is used for forming amides from acyl chlorides and amines?

Nucleophilic addition/elimination.

What are the two functional groups of amino acids?

NH2 and COOH (amine and carboxylic acid).

Are α-amino acids chiral? Why?

Yes, one carbon has 4 different substituents.

Except glycine, where R = H.

Define a zwitterion.

Ions which have both a permanent positive and negative charge, but are neutral overall.

What property must a carbon atom have for the molecule to display optical isomerism about that carbon atom?

4 different substituents attached to one carbon atom.

What are the similarities and differences between two optical isomers?

Same atoms and bonds, but they are non-superimposable mirror images of one another. NOT IDENTICAL in chemical properties necessarily.

Differ in the way they rotate plane polarised light - rotate plane of polarisation by the same angle but in different directions.

What word is used to describe optically active molecules?

Chiral

What is condensation in terms of polymers?

Small molecule eliminated (usually H2O) to form a larger molecule.

How many monomers are condensation polymers usually formed from?

Two (2)

What properties do these monomers forming condensation polymers have?

Each has two functional groups.

Examples of condensation polymers?

Polyesters

Polyamides

Polypeptides

What molecule is eliminated in formation of a polyester?

H2O

What are the two monomers which form a polyester (generic names and structures)?

Diol and dicarboxylic acid or a molecule with both alcohol and a carboxylic acid functional groups.

What is Terylene used for?

As a fibre for making clothes.

Which molecule is eliminated when a polyamide is formed?

H2O

What are the two monomers used to form a polyamide (generic names and structures)?

Diamine and dicarboxylic acid.

Examples of polyamides?

Nylon, Kevlar

If you are making Nylon in the lab, what monomers would you use and why? What molecule is eliminated?

Use hexane-1,6-diacyl chloride as the rate of reaction is much faster. HCl is eliminated.

What is Kevlar used for?

In body armour (bullet proof vests, stab vests), helmets (e.g. F1 drivers’), oven gloves.

Why are poly(alkenes) not biodegradable?

Non-polar C-H and C-C bonds.

Why is it bad to burn poly(alkenes)?

Releases CO2, CO, C (soot) and other toxic

chemicals from monomers.

Where do most poly(alkenes) end up?

Landfill Sites

Why can condensation polymers be broken down?

They have polar bonds.