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Alkanes, Alkenes, Arenes Theory
Terms in this set (28)
Define homolytic fission
It is the breaking of a covalent bond where bonding electrons are given equally to each atom, resulting in the formation of radicals.
Define heterolytic fission
It is the breaking of a covalent bond where both bonding electrons are given to the more electronegative bonding atom, resulting in the formation of ions.
Define free radical
It is a species containing at least one unpaired electron
Define electrophile E+
It is an electron deficient species that accepts an electron pair to form a covalent bond. It has a positively charged center or induced partial positive charge.
It is known as a Lewis acid.
Isomers are molecules with the same molecular formula but different arrangement of atoms (in terms of structure or space).
Define delocalisation/ resonance effect
When p orbitals on 3 or more adjacent atoms overlap, a π electron cloud is formed where electrons can spread out and have less repulsion, resulting in a molecule that has lower energy and is more stable.
Define inductive effect
It arises from the polarisation of electron density in a σ bond due to electronegativity differences between atoms in the bond.
The more electronegative X will draw electron density away from Y, resulting in a partial positive charge on Y. (electron-withdrawing group X)
The electron rich W will donate its electron density to Z, resulting in partial negative charge on Z. (electron-donating group W)
The greater the distance atoms are from the electronegative atom, the weaker the effect.
Define steric effect
Steric effect arises from the repulsion between electron clouds of groups. More groups and large bulky groups hinder the approach of reactants for effective collision, hindering the rate of reaction, causing steric hindrance.
Explain what is meant by a chiral centre.
A chiral centre is any atom with a tetrahedral geometry around it that is bonded to 4 different groups.
Explain why 2-bromobutane is the major product of FRS between butane and Br2 instead of 1-bromobutane.
Radical A has 2 electron donating alkyl groups attached to C● while Radical B only has 1 electron donating alkyl group attached to C●. Therefore radical A is more stable than radical B and forms in larger proportion which leads to formation of 2-bromobutane. This is despite butane having a smaller number of 4 secondary hydrogen atoms that can be substituted to form 2-bromobutane than 6 primary hydrogen atoms that can be substituted to form 1-bromobutane.
Explain why alkanes are unreactive in general.
1. The similar electronegativities between C and H atoms make the molecule non-polar.
2. The covalent C-H and C-C bonds are also relatively strong and require a lot of energy to break.
Explain why branched alkanes have lower boiling points than straight chained alkanes.
Alkane molecules are nonpolar and held together by weak dispersion forces. Straight chained alkanes have a larger surface area of which electron clouds can be distorted, so induced dipole moments form more easily resulting in stronger dispersion forces between molecules compared to branched alkanes. The stronger dispersion forces need more energy to be overcomed, so straight chain alkanes have higher boiling points while branched chain alkanes have lower boiling points.
Describe the environmental consequences from the incomplete combustion of hydrocarbons/ petroleum fuel
Unburnt hydrocarbons and carbon monoxide arise from incomplete combustion of fuel.
Carbon monoxide binds irreversibly with haemoglobin in blood (KIV: Transition Elements), reducing the oxygen carrying capacity of haemoglobin and hence less oxygen carried in blood to the rest of the body. This may lead to lack of oxygen in organs and eventual death.
Unburnt hydrocarbons in the presence of sunlight can react with oxygen to form oxides of nitrogen that mix with ozone and other gases to form photochemical smog that can cause lung damage.
Explain how oxides of nitrogen are formed in a car engine and their harmful effects on the environment
Under the high temperature and pressure conditions of the internal combustion engine, atmospheric nitrogen reacts with oxygen to form NO and NO2.
N2 + O2 → 2NO
2NO + O2 → 2NO2
1. Oxides of nitrogen mix with the ozone and other gases to form chemical smog that can cause lung damage.
2. NO2 can dissolve in rain water to form acid rain which increases acidity of the soil making farm land unsuitable for agriculture, cause corrosion to buildings and increase acidity of water bodies which affects marine life.
Explain what catalytic converters do
Catalytic converters reduce harmful unburnt hydrocarbons, CO, and oxides of nitrogen to harmless CO2, N2 and water vapour.
2NO + 2CO → N2 + 2CO2
2CO + O2 → 2CO2
CxHy + (x+y/4)O2 → xCO2 + y/2H2O
Explain how cis-trans isomerism arises in alkenes/ring structures.
There is restricted rotation due to the presence of a π bond/ about single forms that form part of the ring and there are non-identical groups attached to each carbon of the C=C/ 2 or more C atoms in the ring.
Explain why cycloalkenes only exist in cis form.
Trans form of cycloalkenes with small rings are not stable due to large ring strain. (only for those w >8 C)
Explain why trans isomers tend to have higher melting point
Trans isomers pack better than the cis isomers due to higher symmtery, so dispersion forces have a larger surface area to intreact and work more effectively compared to in cis isomers. Hence, more energy is required to overcome the stronger dispersion forces between trans isomer molecules, resulting in higher melting point.
Explain why cis isomers tend to have higher boiling point
only applicable for cis isomers with non-alkyl groups
In cis isomers, the polar C-𐀠 bond results in dipole moment that does not cancel out, hence producing an overall dipole moment for the molecule. Thus, the cis isomer is polar and form permanent dipole-permanent dipole interactions on top of dispersion forces between molecules.
In trans isomers, the dipole moments cancel out such that there is no overall dipole moment for the molecule. Thus, the trans isomer is nonpolar and only forms weak dispersion forces between molecules.
Hence, more energy is required to overcome the stronger forces of attraction between cis isomers than trans isomers, leading to a higher boiling point.
Explain why despite having chiral centres, a molecule does not exhibit enantiomerism
There is an internal plane of symmetry in the molecule, so its mirror image is superimposable onto itself.
Explain why the major product formed from alkene is the one w secondary added Cl?
The H+ electrophile attacks the C=C to form 2 carbocations. Carbocation intermediate B has 2 electron donating alkyl groups attached to C+ while carbocation intermediate A has only 1 electron donating alkyl group attached C+.Therefore, carbocation intermediate B is more stable than carbocation intermediate A, and forms in larger proportions leading to formation of 2-chloropropane as the major product.
Explain why the product formed from the reaction of but-2-ene with HBr is optically inactive.
But-2-ene contains a chiral centre. The carbocation intermediate is trigonal planar about the sp2 hybridised electron deficient carbon. There are equal chances of C+ being attacked from the top or bottom of the plane by Br- in the second step of Electrophilic Addition. Therefore, there are equal chances of forming either enantiomer, resulting in a racemic mixture with equal proportions of each enantiomer. The 2 enantiomers will rotate plane polarised light at the same angle but in opposite directions, so the racemic mixture will be optically inactive
Explain whether benzene or alkene is more reactive towards electrophiles.
Alkenes are more reactive than benzene. Delocalisation of π electrons in the benzene ring makes π electrons less susceptible to attack by electrophile compared to π electrons localised in C=C.
Explain why benzene prefers to undergo substitution rather than addition reaction.
Benzene is electron-rich and attracts electrophiles, however, as a result of resonance stabilisation due to the delocalisation of π electrons in the benzene ring a significant amount of energy is needed to break the stable structure for addition reaction to take place, so benzene undergoes substitution.
Explain why electrophilic substitution of halogens need to be anhydrous
In the presence of water, FeX or AlX will accept lone electron pairs from water instead of the halogen when generating the strong electrophile for the reaction to proceed.
Describe the role of FeX or AlX in electrophilic substitution
They act as the Lewis acid by accepting the lone pair of electrons from X2 to generate the strong electrophile to activate the resonance stabilised benzene ring as well as a catalyst since it is regenerated at the end of the reaction.
Explain why nitration of methylbenzene requires a lower temperature than nitration of benzene.
Methylbenzene has an electron-donating alkyl group that increases the electron density of the benzene ring making it more reactive than benzene towards electrophilic attack. Hence, milder conditions are required for the reaction to proceed.
Explain why there is no one major product in the mononitration of 1,2-dimethylbenzene.
It has 2 similar activating groups which direct incoming groups to all 4 different positions on the benzene ring.
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