Chemistry paper 2

Describe how you would distinguish between two enantiomers.

Describe how you would distinguish between two enantiomers.

• polarised light

• Enantiomers would rotate light in opposite directions

Explain how a reaction produces a racemic mixture

• planar

• Attack from either side

• With equal probability/ produces equal amounts of the two enantiomers

State the relationship between two chiral molecules with the same structural formula

Non super imposable mirror images

Explain why 3-aminopentane is a stronger base than ammonia

in 3-aminopentane,

• lone pair on N more available

• Because of alkyl electron pushing/ inductive effect

1,6-Diaminohexane can also be formed in a two-stage synthesis starting from 1,4 dibromobutane.

Suggest the reagent and a condition for each stage in this alternative synthesis.

stage 1

Reagent - KCN

Condition - aqueous alcohol

Stage 2

Reagent - H2

Condition - Ni or Pt or Pd

Primary amines can be prepared by the reaction of halogenoalkanes with ammonia or by the reduction of nitriles.

Justify the statement that it is better to prepare primary amines from nitriles rather than from halogenoalkanes.

With nitriles

• higher atom economy/ no further reaction

With halogenoalkane

• lower atom economy/ further reactions

What is the solvent used for NMR analysis and why is it suitable?

tetramethylsilane (TMS)

• non toxic

• Does not react with the sample

• Easily separated from the sample molecule due to its low boiling point

• Produces one strong, sharp absorption peak on the spectrum

properties of CFCs

• inert

• Non flammable

• Non toxic

Uses of CFCs

• refrigerators

• Propellants in aerosols

• As solvents for dry cleaning

What are commonly used as an alternative to CFCs?

Hydrofluorocarbons (HFCs)

Ozone depletion equations

Cl • + O3 —> ClO • + O2

ClO • + O3 —> 2O2 + Cl •

Overall : O3 —> 3O2

Uses of quaternary ammonium salts

Cleaning products

• conditioners for hair

• Fabric softeners

What do quaternary ammonium salts act as in industry?

Cationic surfactants

• lower surface tension

• Allow wetting to take place

• cationic due to positive charge on nitrogen atom

Conditions for catalytic cracking

• zeolite catalyst

• Slight pressure

2A + B —> 3C + D

Justify the statement that adding more water to the equilibrium mixture will lower the amount of A in the mixture

• all concentrations fall

• Equilibrium moves with more moles

• To oppose the decrease in concentration

The enthalpy of hydrogenation of cyclohexa- 1,3 - diene is not exactly double that of cyclohexene

Suggest a value for the entropy of hydrogenation of cyclohexa-1,3-diene and justify your value

~ -200 to -250 KJ mol-1

• double bonds separated by one single bond

• Allows some delocalisation

Define stereoisomerism

compounds that have the same atoms connected to each other, however are arranged differently in space

What are the CIP priority rules for E-Z isomerism

• for a single atom, a higher atomic mass gives a molecule higher priority

• For groups of atom look at the atom directly bonded to the carbons in the double bond

Higher priority same side = Z

Opposite sides = E

Process of fractional distillation

1. Heated till evaporated

2. Into fractional distillation column

3. Condenses in colour at different boiling points

4. Removed at fractions

Conditions of thermal cracking

• high temperatures (500-950C)

• High pressure (<7000 kPa)

Products of thermal cracking

Mainly alkenes

Conditions of catalytic cracking

• high temperature

• Slight pressure

• Zeolite catalyst

Products of catalytic cracking

• branched or short chain alkanes

• Aromatic hydrocarbons

Economic reasons for cracking alkanes

Higher demand/ more uses for shorter chain hydrocarbons

Equation for combustion to show formation of SO2

S(s) + O2 (g) —> SO2 (g)

Equation for combustion to show formation of NOx

N2 + O2 → NOx

Define nucleophile

An atom or compound with a negative or partial negative charge able to form a covalent bond (Donating a pair of electrons)

effect of carbon - halogen bond enthalpy on rate of reaction

Down the group

Decreases

As size of halogen atom increases, bond length increases and reactivity increases

what are the problems from chlorofluorocarbon’s being released into the atmosphere

• broken down by UV light

• Cause formation of chlorine radicals

• Radicals react with ozone

• Break down ozone layer

• Harmful UV light can get to earth

Describe the bonding in a C=C of alkene

One sigma and one pi bond

(Alkanes only have one sigma)

How would you describe a double bond?

An area of high electron density

What is an electrophile?

An atom (or group of atoms) that is attracted to an area of high electron density

Why is a product major or minor in electrophilic addition of alkenes?

• The major product is always formed from the most stable carbocation intermediate

• The stability depends on the group surrounding the positive carbon atom

• Alkyl groups donate electron density to stabilise the ion

• By the photoinductive effect

why are addition polymers unreactive?

The polyalkenes Which are formed contains single bonds only and the main carbon chain is nonpolar so are unreactive

Nature of intermolecular forces between molecules of polyalkanes

• intermolecular forces determine the properties

• The long chains are held together by van der waals forces as the chains are nonpolar

Hydration of an alkene to form an alcohol conditions

• steam

• Acid catalyst - concentrated phosphoric acid

Define biofuel

• Fuel made from living material after it died

Conditions to the fermentation of glucose

• 35C

• 1 atm

• Yeast catalyst

• Aqueous

• Anaerobic

Explain the solubility of an alcohol and how carbon chain length effects it

• solubility falls as the length of the hydrocarbon chain in the alcohol increases

Explain the link between absorption of infrared radiation by bonds in carbon dioxide, methane and water vapour and global warming

• greenhouse gases bonds absorb infrared radiation

• This increases the kinetic energy causing the gases to heat up

• As a result, the temperature of the atmosphere increases which intern warms the Earth

What is the fingerprint region of an IR spec?

Contains tiny differences from species to species allowing it to be identified

Define optical isomer

Physical and chemical properties identical but they differ in ability to rotate plane polarised light

Define the term enantiomer

Compounds with a chiral centre exist as two optical isomers which are known as enantiomers

(Non-super imposed mirror images)

What is optical activity?

The ability of a single optical isomer to rotate the plane of polarisation of plane polarised monochromatic light in molecules containing a single chiral centre

What is racemic mixture?

Is a mixture in which there are equal amounts of enantiomers present in a solution

• Cancel each other out

• Plane polarised light remains the same

Compare the melting and boiling points of carbonyls with similar chain length alcohols

• Alcohols contain hydrogen bonds, carbonyls don’t

• Carbonyls contain permanent dipole - dipole forces

• Hydrogen bonds are stronger

• Therefore carbonyls have lower boiling point

Why does nucleophilic addition a asymmetrical carbonyl will produce a racemic mixture

• C=O group is planar so the CN- ion can attack from either above or below

• Equal probability = equal amounts of enantiomers

Uses of esters

• flavouring

• Perfumes

• Solvents

• Plasticisers

Reaction which forms esters

Condensation reaction

Hydrolysis of ester in acidic conditions equation

Ester + water <> carboxylic acid + alcohol

(Dilute acid catalyst)

Hydrolysis of ester in alkali conditions equation

Ester + NaOH —> alcohol + carboxylate salt

Acidification of salt = carboxylic acid

Compare the byproduct of the reaction of acyl chloride and acid anhydrides in the manufacture of aspirin

Acyl chloride

• faster and exothermic

• Produces highly corrosive hydrogen chloride gas

Acid anhydride

• Cheaper to produce

• Less reactive so can be controlled easily

• Produces less corrosive carboxylic acid

Why in recrystallisation of a solid (preparation of a pure organic solid) do you dissolve in the minimum volume of hot solvent?

• minimum - avoid loss of product

• To dissolve both the organic solid and impurities

Why in recrystallisation of a solid (preparation of a pure organic solid) do you filter the solution?

• to remove any solid impurities remaining in the solution

Why in recrystallisation of a solid (preparation of a pure organic solid) do you cool the filtrate in ice?

• Solid crystallises out and leaves behind impurities in solution

Why in recrystallisation of a solid (preparation of a pure organic solid) Do you wash the crystals with cold solvent and dry the crystals using absorbent paper?

• to remove any excess solution/ impurities

Explain how to measure the melting point of a substance and how its melting point relates to its purity

• melting point test using a melt station

• Pure substances have a sharp well defined melting point

• Impurities tend to lower the boiling point

Explain why in some cases, water baths cannot be used and oil or sand baths need to be used in its place

So temperatures above 100°C can be reached

Why do temperatures need to be kept low when certain reactions are being carried out such as nitration of benzene derivatives or oxidation primary alcohol?

As temperature increases, there is a higher chance of getting more than one e.g. nitro group onto the benzene ring or not to over oxidise to a carboxylic acid

What can aromatic amines be used in?

The manufacture of dyes

Azo dyes

Functional Group: R-N=N-R

Why are amines strong bases?

• positive inductive effect

• Alkyl group donate electron density to the N causing its lone pair of electrons to become more available

What are polyesters made from?

• dicarboxylic acids/ diacyl chlorides

• Diols

What are polyamides made from?

• dicarboxylic acids/diacyl chlorides

• Diamines

If a carboxylic acid is involved in a condensation reaction to former condensation polymer, what is released?

water

If an acyl chloride is involved in a condensation reaction to form a condensation polymer what is lost?

• hydrogen chloride

What is terylene and what is it used for?

• polyester

• Benzene - 1,4 - dicarboxylic acid

• Ethane- 1,2 - diol

• Used to make clothing, tire cords and sails for boats

What is nylon 6,6 and what is it used for?

• Polyamide

• 1,6 diaminohexane

• Hexanedioic acid

• Used to make clothing, rope, carpets and parachutes

What is Kevlar and what is it used for?

• polyamide

• 1,4 diaminobenzene

• Benzene-1,4- carboxylic acid

• Used for making bullet proof vests and car tires

What IMF in ester links?

Permanent dipole- dipole forces

What IMF in amide links?

• hydrogen bonding

Explain the biodegradability of Addition polymers versus Condensation polymers

Addition polymers - non biodegradable

Condensation polymers - biodegradable via hydrolysis (breaks ester and amide bonds)

What are the advantages and disadvantages of landfill?

+

• cheap and easy

• uses existing infrastructure

-

• Uses land which is becoming scarce

• releases methane as the waste decomposes

What are the advantages and disadvantages of incineration?

+

• Can be used for energy production

• reduces landfill

-

• Releases toxic gases and carbon dioxide which causes atmospheric and health issues

What are the advantages and disadvantages of recycling polymers?

+

• sustainable

• saves non-renewable raw material

• some plastic can simply be reformed

-

• plastics can only be recycled by type

• Thermoset plastics are hard to recycle

Explain why TMS is a suitable substance to use as a standard for NMR

• gives a single sharp absorption peak at zero

• All other peaks appear as a shift away from the reference peak

Explain the choice of solvent for H NMR

• non-hydrogen containing solvent

• So doesn’t produce any peaks on the spectrum

What is thin layer chromatography?

• A plate is coated with a solid and a solvent moves up the plate

• Mobile phase - non polar (alkane) or polar (water or alcohol)

• Stationary phase - thin metal sheet coated in alumina (Al2O3) or silica (SiO2)

What to do if the sample is not coloured in thin layer chromatography?

Locate them using - ninhydrin, UV light or iodine vapour

Draw around spots in pencil

What is column chromatography?

A column is packed with a solid and dissolvent moves down the column

• Sample flows through the colon due to gravity

• Components with greater attraction to stationary phase take longer to flow through the column

• Stationary phase - inert solid ( alumina or silica gel)

What is gas chromatography?

A column is packed with a solid or with a solid coated by liquid and a gas is passed through the column under pressure at high temperature

• used for gases, volatile liquids and solids in vapour form

• When reaches detector retention times are recorded

What is the mobile phase?

Solvent that carries the samples

What is the stationary phase?

Material in which the mobile phase travels through

Explain how a structural feature of this enzyme enables it to catalyse the hydrolysis of isomer G but not the hydrolysis of isomer F

• enzyme has an active site

• The g enantiomer has the correct stereo chemistry

  or

• Complementary shape

State why mass spectrometry using electrospray ionisation is not a suitable method to distinguish between the isomers

All have same Mr

Explain why KCN followed by dilute acid is used as opposed to just HCN

• HCN is an extremely poisonous gas

• KN isn’t and the dilute acid supplies the H+ ions

Describe saponification

• vegetable oils and animal fats can be hydrolysed in alkaline conditions with aqueous sodium hydroxide to form soaps

Give a reason why carbon dioxide absorbs infrared radiation

C=O bonds vibrate at the same frequency as infrared radiation

Suggest why chemists usually aim to design production methods with fewer steps and with a higher percentage atom economy

Fewer steps - less energy used or better yield or reduces practical losses

Higher percentage atom economy - Less waste or less pollution Or maximises the use of all materials in the process/saves resources

State why the ozone layer is beneficial for living organisms

Absorbs harmful UV light

State how chlorofluorocarbons form chlorine atoms in the upper atmosphere

C-Cl bonds broken

What is the mechanism for esterification?

Nucleophilic addition - elimination