s&r topic 1

Question 1

alkanes

• C-C hybridisation?
• bond angle?
• describe how C-C bond rotation is allowed

Answer 1

• carbons are sp3 hybridised, can form 4 sigma bonds
• 109.5°
• sp3 orbitals overlap head on head, they have cylindrical symmetry about the internuclear axis so bond can rotate

Question 2

alkene

• C-C hybridisation?
• bond angle?
• describe how the double bond is formed

Answer 2

• carbons are sp2 hybridised, can form 3 sigma bonds
• 120°
• when sp2 hybridisation occurs, there is a left over p orbital on each carbon which are at 90° to the trigonal plane of sp2 hybrids, p orbitals overlap

Question 3

alkyne

• C-C hybridisation?
• bond angle?
• describe the pi-structure of the C-C bond

Answer 3

• carbons are sp hybridised, can form two sigma bonds
• 180°
• two left over p orbitals on each C, pi-bonds form, one with electron density above and below the bond
  one with electron density either side of the bond

Question 4

how does C-C bond length change between alkanes, alkenes and alkynes

Answer 4

alkanes have the longest bond length and alkynes have the shortest

Question 5

define conformation isomers

Answer 5

molecules that are related by rotation around their single bonds

Question 6

describe the saw horse projection

Answer 6

elongated C-C bond with the R groups appearing staggered, not overlapping on the page

Question 7

describe the Newman projection

Answer 7

looking down the C-C axis, R groups are overlapping

Question 8

what is a dihedral angle?

Answer 8

in a Newman projection, it is the angle between the substituents of the central atom behind and in front

Question 9

what is a staggered conformation

Answer 9

substituents are as far apart as possible

dihedral angle are all equal whilst being at maximums

Question 10

what is an eclipsed conformation

Answer 10

substituents are directly lined up, in front of each other

dihedral angles are 0

see topic1 vid2 for illustrations

Question 11

what is more stable, staggered or eclipsed conformation

what causes this energy difference/difference in stability

Answer 11

staggered is more stable

eclipsed conformations are higher in energy, but only by. a small amount

energy difference/difference in stability caused by torsional strain, in eclipsed conformation, substituents are closer together, increasing repulsion between them, therefore increasing torsional strain

Question 12

what is torsional strain

Answer 12

repulsion that arises from the repulsion between atoms or groups of atoms when a molecule is rotated around a sigma bond

can be lessened by rotating around the sigma bond

Question 13

why are staggered conformations more stable than eclipsed?

Answer 13

in staggered conformation, there is orbital overlap which leads to orbital mixing

the σ(C-H) orbital on one C lines up perfectly which the σ*(C-H) orbital on the adjacent C, they mix to form a new slightly more stable bonding and a new slightly less stable anti-bonding molecule

Question 14

at room temp, what is able to happen between the eclipsed and staggered conformations

Answer 14

at 25°, there is enough energy to overcome the small energy difference between them

the molecules will be rotating very quickly and swapping between the two conformations and all of the ones in-between

Question 15

how does a molecule (an alkane) switch between conformers

Answer 15

by rotating by 60° around a sigma bond

each conformer differs by ±60°

Question 16

name the conformer with a dihedral angle of:

• 0°
• ±60°
• ±120°
• 180°

which one is higher in energy

Answer 16

• synperiplanar - highest energy
• synclinal or gauche
• anticlinal
• antiperiplanar - lowest in energy

Question 17

what is the name given to conformers with dihedral angles between -30 and 30 or -150 and 150

Answer 17

periplanar

synperiplanar - if corresponding groups are in the same hemisphere
antiperiplanar - if corresponding groups are in different hemispheres

Question 18

what name is given to conformers with dihedral angles between ±30° to ±150°

Answer 18

clinal

synclinal/gauche - if corresponding groups are in the same hemisphere
anticlinal - if corresponding groups are in different hemispheres

Question 19

name the lowest energy conformer

Answer 19

antiperiplanar

Question 20

name the highest energy conformer

Answer 20

synperiplanar

see topic1 vid3 for energy diagram of conformers

Question 21

define conformer

name the two conformers

Answer 21

arrangements of molecules that correspond to a minimum of the potential energy

synclinal/gauche and antiperiplanar are at minima on the energy diagram so are conformers - they are the 2 staggered conformations

Question 22

name the conformations at energetic maxima

in conformation equilibrium, what are these considered

Answer 22

synperiplanar and anticlinal

these conformations are transition states between the energetic minima/conformers

they are higher in energy = have a shorter life span

Question 23

define steric strain

Answer 23

repulsion between two atoms or groups of atoms when the distance between them is decreased/they become closer together

steric strain cannot be lessened by rotating the bonds directly attached to them