Week 7

Question 1

Carbon’s electrons

Answer 1

Shares 4 electrons to form 4 covalent bonds
* 2 x 2s electrons + 2 x 2p electrons are available
* But these orbitals (2s + 2p) mix (hybridise) first

Question 2

sp3 hybridisation

Answer 2

If the 2s orbital mixes with all 3 2p orbitals…
* (1 x s) + (3 x p) = 4 x sp3
* I.e. 4 x identical orbitals -> forms 4 x single (covalent) bonds

Question 3

sp2 hybridisation

Answer 3

If the 2s orbital only mixes with 2 2p orbitals..,
* (1 x s) + (2 x p) = 3 x sp2 (+1 x p-orbital remains unhybridized)
* I.e 3 x identical hybridised orbitals + 1 different one -> forms 3 x single bonds + 1 x double bond

Question 4

Properties of alkanes

Answer 4

• Chemically inert (i.e. don’t do a whole lot!)
• Do react with oxygen (combustion) -> used as fuels
• short-chain alkanes burn efficiently CO2 + H2O
• bigger alkanes don’t burn as well ( sooty flame)

Question 5

Constitutional isomers

Answer 5

Isomers that differ in the nature or sequence of bonding
* Within a homologous sequence of alkanes, the number of constitutional isomers increases rapidly

Question 6

Stereoisomers

Answer 6

Isomers which differ in arrangement of groups in space (but same nature and sequence of bonding)
Two subsets:
* configurational
* conformational

Question 7

Configurational isomers

Answer 7

Interconverted only by breaking and remaking bonds.
* This process normally requires a lot of energy so does not happen at room temperature

Question 8

Conformational isomers (conformers)

Answer 8

Differ by rotation about a single (C-C) bond
* Easy at room temperature so conformational isomers are not normally separable

Question 9

A double bond is includes a

Answer 9

sigma bond and a pi bond

Question 10

To rotate around the C–C axis would require…

Answer 10

The p-bond to break
* takes 128 kJ mol-1 and does not occur without significant energy input

Question 11

Double bond isomers

Answer 11

If both ends of the C=C bond each have two different groups attached then:
* two diastereoisomers result

Question 12

Z or E diasteroisomers

Answer 12

• If high priority groups on same side of C=C ⟹ Z double bond
• If high priority groups on opposite sides of C=C ⟹ E double bond

Question 13

The higher the atomic number of the atom attached…

Answer 13

the higher the priority

Question 14

Rhodopsin

Answer 14

Pigment found within the retina of our eyes
* Within it is retinal - respond to light coming in

Question 15

In ‘straight chain’ alkanes, rotation about C-C bonds is ____ at room temperature

Answer 15

Rapid
* Differences in energy arise from steric interactions

Question 16

Configurational isomers
can also arise when…

Answer 16

an sp3 atom has 4 different bonding partners

Question 17

Chirality

Answer 17

An object is chiral if it is NOT superimposable upon its mirror image

Question 18

Enantiomers

Answer 18

• Are mirror images of each other
• Are not superimposable (i.e. not the same).
• Commonly contain one or more stereocenters

Question 19

stereocenter

Answer 19

stereogenic centre = chiral centre
stereocenter = carbon atom with four different groups attached

Question 20

racemic mixture (racemate)

Answer 20

50:50 mixture of both enantiomers

Question 21

Each individual chiral centre has two possible arrangements

Answer 21

Enantiomers labelled as R or S

Question 22

When Does Chirality Matter?

Answer 22

Chiral molecules occur frequently with increasing molecular size
* Almost all the physical properties of a pair of enantiomers are identical (melting point, boiling point, solubility, NMR, IR etc.)
* Behave in an identical way when interacting with achiral objects or reagents
* Will interact & react differently with other chiral molecules

Question 23

Almost all biological catalysis by enzymes is selective for a particular enantiomer.

Answer 23

True,
Almost all proteins and enzymes are chiral

Question 24

Measuring optical activity

Answer 24

When plane polarised light is passed through a solution of one pure enantiomer of a compound, the plane of polarisation is rotated:
* Enantiomer is said to be optically active
* Amount of rotation (⍺) is characteristic of the enantiomer

Question 25

One enantiomer causes clockwise rotation, the other enantiomer rotates the light an equal amount in the opposite direction

Answer 25

• rotation in a clockwise direction is labelled (+)
• rotation anti-clockwise is labelled (-)
• racemic mixtures give an overall rotation of zero

Question 26

Diasteroisomers

Answer 26

Configurational isomers that are NOT enantiomers
* All achiral config. isomers are diasteromer

Question 27

each chiral centre has…

Answer 27

2 possible arrangements