Advanced Higher Chemistry- Organic

when do molecular orbitals form

when atomic orbitals combine

the combination of two atomic orbitals will result in

the formation of a bonding orbital and an anti bonding orbital

what is a bonding molecular orbital

a molecular orbital which has a lower energy level than either of the two atomic orbitals forming it

what is antibonding orbital

a molecular orbital that has a higher energy than either of the two atomic orbitals forming it

the maximum number of electrons occupying a molecular orbital is

2

the basis of bonding between atoms

the attraction of the positively charged nuclei and the negatively charged electrons in the bonding molecular orbital

how are sigma bonds formed and properties

end on overlap, stronger than pi bonds

how are pi bonds formed

side on overlap, weaker than sigma bonds

what is hybridisation

the mixing of atomic orbitals to create a new set of hybrid orbitals

hybrid orbitals are

degenerate

bonding in alkanes

one 2s orbital and three 2p orbitals combine to form sp3 hybrid orbitals, these have a tetrahederal shape

bonding in alkenes

the mixing of one 2s orbital and two 2p orbitals to form sp2 hybrid orbitals, which have a trigonal planar shape.

bonding in alkynes

one 2s orbital and one 2p orbital combine to form sp hybrid orbitals, resulting in a linear shape.

bonding in benzene

six carbon atoms are arranged in a cylic structure with sigma bonds between carbons. the unhybridised p orbitals form a delocalised pi system perpendicular to the plane of the sigma bonds

in a non-polar covalent bond the bonding molecular orbital is

symmetrical about the midpoint between two atoms

in a polar covalent bond the bonding molecular orbital is

asymmetrical favouring the more electronegative atom

ionic character

a measure of the difference in electronegativity between two atoms- the greater the distance the greater the ionic character

covalent character

a measure of the difference in the electronegativity of two atoms- the smaller the difference the greater the covalent character

most organic compounds are colourless because

the light absorbed is outside the visible range of the EM spectrum

electrons fill the ____ molecular orbitals first

bonding- as they have a lower energy level than antibonding orbitals

under normal conditions the antibonding orbital is

empty

what does HOMO stand for

Highest Occupied Molecular Orbital, the highest bonding orbital containing electrons

what does LUMO stand for

Lowest Unoccupied Molecular Orbital. the lowest antibonding molecular orbital- does not contain electrons

In organic molecules which only contain single carbon bonds with no non-bonding electrons the HOMO is

the sigma bonding orbital

In organic molecules only containing single carbon bonds with no non-bonding electrons the LUMO is

the sigma antibonding orbital

when the energy gap between the HOMO and the LUMO is large

the promotion of electrons results in UV light being absorbed thus making the compound colourless

coloured organic compounds are

conjugated systems

what is a conjugated system

a system of adjacent unhybridised p orbitals that overlap side on to form a molecular orbital across a number of carbon atoms- alternating single and double bonds

in a conjugated system electrons are

delocalised

as the conjugation in a molecule increases

the energy gap between the HOMO and the LUMO decreases

what is a chromophore

the group of atoms within a molecule responsible for the absorption of visible light

structural isomer

the atoms are bonded together in a different order in each isomer

stereoisomers

the order of bonding is the same, but the spatial arrangement of the atoms is different in each isomer

two types of stereoisomers

geometric and optical

geometric isomerism occurs in compounds where

there is a lack of free rotation around a carbon with a double bond and two different groups attached to the carbon with the lack of free rotation

cis isomer

both groups are on the same side

trans isomer

groups are on opposite sides

optical isomers (enantiomers)

asymmetric, non superimposable mirror images of each other. there are four different groups surrounded around a central carbon

differences in physical properties between enantiomers

same physical properties, except their effect on plane polarised light

what happens when plane polarised light is passed through a solution containing one enantiomer

the plane polarised light is rotated through a certain angle

what happens when a solution of one enantiomer is replaced by an equimolar solution of the opposite enantiomer

the plane polarised light is rotated in the same angle in the oposite direction

what is a racemic mixture

a mixture of equal amounts of two enantiomers, optically inactive- rotational effect cancels out

homolytic fission

when a covalent bond breaks and equally splits electrons between each atom forming two neutral radicals, non-polar bonds are broken

why are reactions involving homolytic fission unsuitable for synthesis

they tend to result in the formation of very complex mixtures of products due to their presence of free radicals

heterolyitic fission

a sigma covalent bond breaks and the atom with highest electronegativity retains the shared electrons forming two oppositely charged ions, polar bonds break

double headed arrows show

two electrons moving so heterolytic fission occuring

single headed arrow

one electron moving, homolytic fission occuring

an arrow drawn with the head pointing between two atoms indicates

a covalent bond will form between the two atoms

nucleophile

electron rich, negatively charged or neutral, act as electron doners

electrophiles

electron poor, postively charged or neutral molecules, electron acceptors

electrophiles are attracted to

atoms bearing a partial or full negative charge

nucleophiles are attracted to

atoms bearing a partial or full positive charge

haloalkane

substituted alkane in which one or more of the hydrogen atoms is replaced with a halogen atom

why do monoalkanes undergo neucleophilic reactions

the carbon in the carbon-halogen bond has a partial positive charge due to the halogen having a greater negativity

monoalkanes and aqueous alkalis undergo nucleophilic substitution to form

alcohols

monoalkanes and alcoholic alkoxides undergo nucleophilic substitution to form

ethers

alcoholic alkoxide

alkali meta (e.g. potassium) added to alcohol

monohaloalkane and potassium/sodium ethanolic cyanide undergo nucleophilic substitution to form

nitriles

ethanolic cyanide

solution of a cyanide compound in ethanol

why is the reaction of a monohaloalkane and ethanolic cyanide useful in synthesis

the carbon chain increases by one due to the carbon contributed by the cyanide

nitriles can be hydrolysed to form

carboxylic acids

monohaloalkanes undergo base induced elimination eith ethanolic metal hydroxide to form

alkenes

SN1 mechanism

a nucleophilic substitution reaction with one species in the rate determining step, following first order kinetic