Topic 20: Organic Chemistry (HL)

Question 1

Sn2 reactions

Answer 1

Nucleophilic substitution in primary halogenoalkanes

• single-step reaction
• rate of reaction is dependent on the concentrations of both reactants
• 2nd order reaction

rate = k[halogenoalkane][nucleophile]

• bimolecular
• stereospecific, where starting reagents differ only in their configuration and are converted into stereoisomeric products

Question 2

Walden inversion

Answer 2

When a chemical species with a sp3
carbon center and tetrahedral
geometry undergoes a backside attack by a nucleophile in a SN2 reaction, a
configurational change occurs

Question 3

Sn1 reactions

Answer 3

Nucleophilic substitution reaction involving tertiary halogenoalkanes.
Two steps.

• rate-determining step, where only the halogenoalkane is involved, ie. the bond of the leaving group breaks, forming a carbocation.
• Reaction is 1st order:

rate = k[halogenoalkane]

• reason why tertiary halogenoalkanes undergoes reactions via Sn1 mechanism is becuase the alkyl groups bonded to a carbocation have a positive inductive effect by stabilizing the ion by donating electron density. In primary halogenoalkane there is only one alkyl group but in tertiary there;s three so it will be more stable.

Question 4

What is the most important factor on determining which substitution mechanism occurs between a nucleophile and a halogenoalkane?

Answer 4

The differences in electronegativity between atoms present in the molecule.

Question 5

List three main factors that affect the rate of nucleophilic substitution.

Answer 5

1) the identity of the halogen - as the carbon-halogen bond gets longer and weaker, the quicker the rate-determining step is completed, the higher the rate of reaction.
- Bond strength, length and electronegativity are important. As you go down the halogen group the strength of the carbon-halogen bond decreases as the atom size increases.

2) the classes of halogenoalkane: primary, secondary or tertiary

3) the choice of solvent
- Sn2 reactions are best in aprotic, polar solvents, because no O-H or N-H groups, ie no hydrogen bond
- Sn1 in protic, polar solvents
- the smaller the nucleophile, the more effective the solvation, ie. less effective at forming the Sn2 intermediate, so Sn1 is favoured.

Question 6

Solvation

Answer 6

Process where solvent molecules surround the dissolved ions.

Question 7

Electrophile

Answer 7

• An electron-deficient species that will accept a pair of electrons
• Acts as a Lewis acid

Question 8

Markovnikov’s Rule

Answer 8

• Used to predict the products of the electrophilic addition of hydrogen halides to unsymmetrical alkenes.
• The hydrogen atom will preferentially bond to the carbon atom of the alkene that is already bonded to the largest number of hydrogen substituents

Question 9

Interhalogens

Answer 9

Compounds in which two or more halogens are combined in a molecule.

• differences in electronegativity between the halogens will result in an eletrophilic region of the molecule and this determines which halogen will attack the pi bond in an addition reaction.

Question 10

Describe the nitration of benzene.

Answer 10

• electrophilic substitution reaction
• nitronium ion electrophile, NO2^+ is generated from a nitrating mixture of sulfuric acid and nitric acid.
• as nitronium ion approaches the delocalised pi electrons of the benzene ring, the nitronium ion is attracted to the ring.
• electrons from the ring are donated to form a new C-N bond and eliminating the aromaticity of the arene
• water acts as a base, deprotonating the carbocation intermediate and restoring the aromaticity

Question 11

Name two reducing agents in the reduction of carboxylic acids

Answer 11

1) lithium aluminium hydride, LiAIH4 - reduces the polar C=O bonds present in carboxylic acids, aldehydes and ketones
2) sodium borohydride, NaBH4

Question 12

General equation for reduction of an aldehyde to a primary alcohol

Answer 12

R-CHO + 2[H] → R-CH2OH

where [H] represents the reducing agent

Question 13

General equation for reduction of ketones to secondary alcohol

Answer 13

R CO R′ + 2[H] → R-CH(OH) R′

Question 14

Reflux

Answer 14

Process in which a reaction mixture is heated under controlled conditions for a period of time. A condenser is used to cool the vapours from volatile solvents and condense them back into the reaction mixture. The process ensures that the temperature remains constant over time and optimal conditions for the reaction are achieved.
clamps

Question 15

Polymerization

Answer 15

Formation of long molecules made up of repeating monomer units.

Question 16

Reaction pathway for aromatic compounds

Answer 16

benzene → nitrobenzene → phenylamine (aniline)

Question 17

Reaction pathway fro polymers

Answer 17

alkene → polymer

Question 18

Stereoisomer

Answer 18

Have an identical molecular formula and bond multiplicity but show different spatial arrangements of the atoms.

Can be divided into two types: conformational isomerism and configurational isomerism

Question 19

Conformational isomer

Answer 19

Can be interconverted by rotation about the sigma bond, without breaking any bonds.

Question 20

Configurational isomer

Answer 20

Can only be interconverted by breaking sigma or pi bond or through rearrangement of the stereocentres.

Question 21

Newman projection

Answer 21

A representation of the 3D structure which shows the conformation of the molecule by looking along the carbon-carbon bond.

Question 22

Torsional strain

Answer 22

Energy difference between the staggered and eclipsed conformations. It’s the result of the repulsion between bonding electrons.

Question 23

Cis-isomers

Answer 23

Have substituents on the same side of the reference plane, which is the carbon=carbon.

Question 24

Trans-isomers

Answer 24

Have substituents on the opposite side of the reference plane.

Question 25

How to assign an E (cis) ir z (trans) configuration?

Answer 25

Atoms bonded to carbon atoms are ordered from highest to lowest atomic number, Z. If both higher priority substituents are on the same side of the double bond, the isomer is Z or trans.

Question 26

Optical isomerism

Answer 26

• Type of configurational isomerism determined by the presence of chiral atoms.
• Can rotate plane-polarized light
• Exist in pairs that are called enantiomers or diastereomers

Question 27

Chiral carbon atom

Answer 27

• also known as stereocentre

- chiral carbon is bonded to four different atoms or group of atoms

Question 28

Enantiomers

Answer 28

• non-superimposable mirror images of each other

- no plane of symmetry

Question 29

Physical properties of optical isomers

Answer 29

• have identical physical properties like boiling and freezing points, viscosity, density and solubility.

Question 30

Chemical properties of optical isomers

Answer 30

• many properties are identical except for their reactions with other optical isomers, often in biological systems, e.g. enzymes, which are chiral and can distinguish between enantiomers of their substance.

Question 31

Diastereomers

Answer 31

• non-superiposable
• do not form mirror images
• two or more stereocentres
• differ in the configuration of at least one centre
• diastereomers with the same general formula have different physicial and chemical properties.