Alkanes

Question 1

Alkanes formula

Answer 1

CnH2n+2

Question 2

Alkane ring formula

Answer 2

CnH2n

Question 3

Alkane ring formula

Answer 3

CnH2n

Question 4

Alkene formula

Answer 4

CnH2n

Question 5

Alkyne formula

Answer 5

CnH2n-2

Question 6

Degree of unsaturation

Answer 6

One degree for every two hydrogen lost relative to the fully saturated parent alkane.

Question 7

Working out DBE with heteroatom replacement

Answer 7

Double bond equivalent

Monovalent atom:
I, Br, Cl, F, by H

O, S, by CH2

N, P, by CH

=> +1 H for each lone pair

0 DBE, linear compound
1 DBE, 1 ring or 1 1 double bond
2 DBE, 2 rings, or 1 ring 1 double bond, 2 double bonds, 1 triple bond
…

Question 8

Carbon orbitals, carbon hybridisation, LCAO

Answer 8

4 atomic orbitals

• 2s
• 3 2p (x,y,z)

Linear combination of atomic orbitals results in hybridisation and four sp3 molecular orbitals, angle = 109.5° average (as they vibrate)
All of the substituents share the electrons equally: form a tetrahedron.
eg methane

C carbons on double bonds:
Trigonal
Unsaturated
Planar
Sp2

Carbons on single bonds:
Tetrahedral
Saturated
Sp3

For each element, the bonding orbital has a big node pointing towards the element, a small node pointing backwards and vice versa for the antibonding orbital (=important to reactivity, alkyl halides).

–> sigma framework

Question 9

Lone pairs and sharing electrons

Answer 9

Are held closer to the nucleus of the parent atoms, therefore exhibit a greater repulsive effect than those in a covalent bond.

Effect on bond angle: not shared equally, example of ammonia and water.

Same effect with inequivalent substituents, eg ethane, H2 vs Me

Question 10

Torsional angle

Answer 10

Angle between groups around the rotating bond.

• 0° -
• 60° -
• 180° -

Question 11

Conformations, Conformational change, conformational equilibrium, favoured conformation

Answer 11

Conformations
Different shapes of molecules of the same compound
As single bonds are free to rotate, and do so constantly at room T

Without breaking or making bonds

PartIcularly stable conformations are called conformers= conformation equilibrium.

Favoured conformations: lowest energy structures.

Question 12

Energy maxima and maxima and conformation, rate of rotation

Answer 12

Energy maxima
Eclipse structures:
- torsional angle :0°
- atomic overlap as high as physically possible
-van der walls interactions maximum

Energy minima
Staggered
- torsional angle: 60°, 180°…
- no groups across a single bond overlapping

Slow rate of rotation = high energy barrier

Question 13

2 types of projections, the conformations, potential energy diagram of n-butane

Answer 13

• sawhorse
• newman: looking down a bond, with a circle

Staggered

• antiperiplanar 180°
• synclinal gauche 60°, for n-butane+

Eclipsed

• Anticlinal 60° but large group eclipses small group, for n-butane+
• sunperiplanar 0°

Potential energy diagram
Antiperiplanar to anticlinal to synclinal to synperiplanar to synclinal to …

Question 14

Stereoelectonics

Answer 14

The way that the way electrons are shared effects shape.

Antibonding and bonding orbitals can share alectrons:
- the bonding overlaps at the back with the antobonding at the front
ie antiperiplanar (180°) relationship yields the maximum orbital overlap of substituents on adjacent carbon centers
-increased bonding interactions results in a lower energy conformation

Maximised interactions in staggered conformations.

Question 15

Conjugation and hyperconjugation

Answer 15

Conjugation: electrons are shared completely and form delocalised bonds

Hyperconjugation: partial sharing of electrons
Possible in bonding and antibonding interactions.

Question 16

Steric effect, hindrance and van der walls repulsions, steric strain

Answer 16

Steric effect
Any effect on a molecule or reaction due to the size of atoms or groups.

Steric hindrance
The higher energy price and slower reaction rate due to the approach of larger atoms or groups in a chemical reaction, compared to a similar reaction involving smaller atoms or groups.
Eg : heavier groups attached to a carbon cause steric hindrance to SN2 reactions of that carbon.

Van der walls repulsion
when same charge particles are close together.
Eg: axial interaction in chair conformation cyclohexane.

Steric strain
An increase in molecular potential energy cause when atoms or groups separated by at least four covalent bonds are forced closer together than their van der walls radii allow.
Eg eclipse conformation of hexane, terminal methyl groups.

Question 17

Rings, planar or 3D, bin size of rings

Answer 17

Conformationally restricted

Cyclopropane is the only flat cycloalkane
If flat:
- ring strain would incurred by forcing the ring to be planar
- cyclopentane would be the lowest in energy

Actually 3D:

• cyclohexane is the most stable
• ring puckering reduces strain

Bin size
3-6 small
6-11/12 medium
11/12-+ big

Question 18

Cyclic conformation, energy difference, types of substituents, potential energy diagram

Answer 18

Chair conformation
Axial and equatorial
From conformer A to B just by twisting bonds
- one in the anticlockwise direction
- change from axial to equatorial and vice versa
Identical E if same substituents, otherwise:
The lowest energy conformer will place the largest group equatorial to minimize the axial interactions
=> 1,3 diaxial interactions
=> 95% Equatorial, but can be locked if bulky substituent

Boat
Twist boat, half chair

Potential energy diagram: chair A, half chair A, twist boat, boat, twist boat, half chair B, chair B

Question 19

Heterolytic fission

Answer 19

The pair of electrons in the bond becomes associated with only one the atoms when the bond breaks.

Question 20

Super acids, pka def, reasons

Answer 20

Acid with a Hamment avid function <12
Very low pka
Not very useful reaction as only the

Ph at which half the acid is protonated and half deprotonated.
-1 = 10x lower

Hyperconjugation
Tertiary carbo cation lower in energy then methyl.
Due to hyperconjugation :
- a single proton does not align with the empty p orbital on C
Another example of stereoelectronic stability

Question 21

Homolytic fission

Answer 21

Free radical chain reaction