unit two - molecular orbitals Flashcards Preview

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Flashcards in unit two - molecular orbitals Deck (26):
1

what does HOMO stand for ?

highest occupied molecular orbital , pi

2

what does LUMO stand for ?

lowest unoccupied molecular orbital σ*

3

what are the 2 anti bonding molecular orbitals ?

π*. and σ*

4

what are the 2 bonding molecular orbitals ?

π and σ

5

what is homo and lumo in single bond molecules and why ?

homo σ and lumo σ* as only sigma bonds present and no π bonds.

6

why are compounds coloured ?

the energy gap between homo and duo corresponds to a wavelength of light within the visible spectrum.

7

why are compounds not coloured ?

the energy gap between homo and lump correspond to a wavelength of light outwith the visible spectrum.

8

what is a conjugated system ?

when the structure has alternating single and double bonds, includes benzene ring.

9

what is a chromophore ?

the part of a molecule that is responsible for colour

10

can a chromophore have a benzene ring ?

yes

11

whats the link between the size of chromophore and energy gap ?

the larger the chromophore the smaller the energy gap between homo and lumo so light of a longer wavelength will be absorbed , more likely to be in visible spectrum.

12

what are the 2 types of stereoisomerism ?

geometric and optical isomerism

13

why does geometric isomerism arise ?

due to a lack of rotation around a bond normally a c-c double bond as this would involve breaking a pi bond.

14

are stereoisomerisms superimposable ?

no

15

whats the 2 groups of geometric isomers ?

trans and cis isomers .

16

what is trans isomer ?

opposite sides of double bond

17

what is cis isomer ?

same side of bond

18

can trans isomers eliminate water and why ?

no because the carboxyl groups will be on opposite sides

19

do geometric isomers have the same physical properties ?

no they are different

20

why do trans isomers have a higher melting point ?

the trans isomer is able to pack more tightly together than cis isomers , this increases the van Der Waals forces between the molecules and hence the melting point is higher.

21

why do cis isomers have a higher boiling point that trans isomers ?

the bonds lie on the same side as the double bond in a cis isomer which results in the molecule itself being polar. However in a trans isomer the bond are on opposite sides of the double bond. , which causes them to cancel each other out and the molecule itself is not polar.

22

what is optical isomerism ?

a molecule has a tetrahedral carbon atom with 4 different groups attached to it .

23

Are optical isomers superimposable and mirror images ?

they are not superimposable but they are mirror images.

24

what is the chiral carbon ?

the carbon in the middle with 4 different groups attached.

25

do optical isomers have different physical properties ?

no they have the same physical properties except they have opposite effects on polarised light.

26

what is a racemic mixture ?

a mixture that contains equal amounts of the 2 optical isomers. It will be optically inactive