s&r topic 1 Flashcards
hydrocarbons and conformations
alkanes
- C-C hybridisation?
- bond angle?
- describe how C-C bond rotation is allowed
- 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
alkene
- C-C hybridisation?
- bond angle?
- describe how the double bond is formed
- 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
alkyne
- C-C hybridisation?
- bond angle?
- describe the pi-structure of the C-C bond
- 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
how does C-C bond length change between alkanes, alkenes and alkynes
alkanes have the longest bond length and alkynes have the shortest
define conformation isomers
molecules that are related by rotation around their single bonds
describe the saw horse projection
elongated C-C bond with the R groups appearing staggered, not overlapping on the page
describe the Newman projection
looking down the C-C axis, R groups are overlapping
what is a dihedral angle?
in a Newman projection, it is the angle between the substituents of the central atom behind and in front
what is a staggered conformation
substituents are as far apart as possible
dihedral angle are all equal whilst being at maximums
what is an eclipsed conformation
substituents are directly lined up, in front of each other
dihedral angles are 0
see topic1 vid2 for illustrations
what is more stable, staggered or eclipsed conformation
what causes this energy difference/difference in stability
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
what is torsional strain
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
why are staggered conformations more stable than eclipsed?
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
at room temp, what is able to happen between the eclipsed and staggered conformations
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
how does a molecule (an alkane) switch between conformers
by rotating by 60° around a sigma bond
each conformer differs by ±60°
name the conformer with a dihedral angle of:
- 0°
- ±60°
- ±120°
- 180°
which one is higher in energy
- synperiplanar - highest energy
- synclinal or gauche
- anticlinal
- antiperiplanar - lowest in energy
what is the name given to conformers with dihedral angles between -30 and 30 or -150 and 150
periplanar
synperiplanar - if corresponding groups are in the same hemisphere
antiperiplanar - if corresponding groups are in different hemispheres
what name is given to conformers with dihedral angles between ±30° to ±150°
clinal
synclinal/gauche - if corresponding groups are in the same hemisphere
anticlinal - if corresponding groups are in different hemispheres
name the lowest energy conformer
antiperiplanar
name the highest energy conformer
synperiplanar
see topic1 vid3 for energy diagram of conformers
define conformer
name the two conformers
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
name the conformations at energetic maxima
in conformation equilibrium, what are these considered
synperiplanar and anticlinal
these conformations are transition states between the energetic minima/conformers
they are higher in energy = have a shorter life span
define steric strain
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
