Flashcards in unit two - molecular orbitals Deck (26):
what does HOMO stand for ?
highest occupied molecular orbital , pi
what does LUMO stand for ?
lowest unoccupied molecular orbital σ*
what are the 2 anti bonding molecular orbitals ?
π*. and σ*
what are the 2 bonding molecular orbitals ?
π and σ
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.
why are compounds coloured ?
the energy gap between homo and duo corresponds to a wavelength of light within the visible spectrum.
why are compounds not coloured ?
the energy gap between homo and lump correspond to a wavelength of light outwith the visible spectrum.
what is a conjugated system ?
when the structure has alternating single and double bonds, includes benzene ring.
what is a chromophore ?
the part of a molecule that is responsible for colour
can a chromophore have a benzene ring ?
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.
what are the 2 types of stereoisomerism ?
geometric and optical isomerism
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.
are stereoisomerisms superimposable ?
whats the 2 groups of geometric isomers ?
trans and cis isomers .
what is trans isomer ?
opposite sides of double bond
what is cis isomer ?
same side of bond
can trans isomers eliminate water and why ?
no because the carboxyl groups will be on opposite sides
do geometric isomers have the same physical properties ?
no they are different
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.
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.
what is optical isomerism ?
a molecule has a tetrahedral carbon atom with 4 different groups attached to it .
Are optical isomers superimposable and mirror images ?
they are not superimposable but they are mirror images.
what is the chiral carbon ?
the carbon in the middle with 4 different groups attached.
do optical isomers have different physical properties ?
no they have the same physical properties except they have opposite effects on polarised light.