9729 organic chem notes

disclaimer: this set of notes is only comprehensive for the following sub-topics of organic chemistry for H2:- alkenes- arenes- halogen derivatives- alcohols & phenols- carbonyl compounds However, the mind maps included are comprehensive of all the organic chemistry reactions in the 9729 syllabus, minus free radical substitution of alkanes.

Physical properties Trend of boiling points down the homologous series (applicable to all):- As the number of carbon atoms increase, the size of the elctron cloud increases, and becomes more polarisable. - The strength of id-id increases, so more energy is required to overcome the id-id, leading to higher boiling points. cis vs trans isomer:- cis isomers have higher boiling points, as they have a net dipole moment- individual dipole moments cancel each other out in trans isomers- cis isomer is a polar molecule held together by pd-pd, while trans isomers are held together by id-id- more energy is required to overcome pd-pd than id-id- trans isomers have better symmetry than cis isomers, allowing them to pack together more efficiently in the solid state- closer packing allows for stronger id-id between molecules, which require more energy to overcome soluble in non-polar solvents- alkenes can form favourable id-id attractions with the solvent insoluble in water and other highly polar solvents- energy released from the id-id formed between alkenes and water molecules are insufficient to compensate for the energy required to overcome the id-id between alkene molecules and hydrogen bonding between water molecules. FAQ Why is XXX the major product?- the __ carbocation has more electron-donating alkyl groups attached to the positively-charged C atom- positive charge on the C atom becomes more dispersed, so the carbocation is more stable, hence (major product) is more likely to be formed Explain why the resulting mixture is optically inactive even though the product contains a chiral carbon.- the carbocation is trigonal planar wrt the positively-charged C atom- The _ nucleophile can attack the positively-charged C atom from either side of the plane with equal probability- equal amounts of both enantiomers are formed, forming a racemic mixture- the 2 enantiomers rotate plane-polarised light by the same extent but in Saytzeff’s Rule: When there is more than one alkene that can be formed from elimination, the major product is the alkene with the greatest number of alkyl groups bonded to C atoms in the C=C double bond. Markovnikov’s Rule: The electrophile is added to the C=C bond in a way such that the most stable carbocation intermediate is produced.

Mechanisms to know: Electrophilic Addition

Resonance : the p orbitals of carbon atoms in a benzene ring overlap sideways with one another, creating a continuous overlap of unhybridised p orbitals throughout the benzene ring. The π electrons delocalise across the benzene ring, creating a delocalised π electron cloud. identical C-C bond lengths- due to the delocalised electron cloud in benzene, all 6 C-C bond lengths are in between the C-C bond length in alkanes and C=C bond length in alkenes- due to partial double bond character of C-C bond in benzene planar molecule- all 6 C atoms are sp2 hybridised = all are trigonal planar shaped = entire molecule is planar substitution reactions- substitution reactions preserve the resonance stability of the benzene ring while addition reactions disrupt it- benzene ring undergoes substitution reactions instead of addition ones as they are more energetically feasible. Physical properties low melting & boiling point- benzene is a non-polar simple molecule, held together by weak id-id soluble in non-polar organic solvents- form favourable id-id interactions with each other insoluble in polar solvents like water- energy released to form id-id between benzene and water molecules is insufficient to compensate for the energy required to overcome the id-id between benzene molecules and strong hydrogen bonding between water molecules FAQ Why is conc. H2SO4 catalyst necessary for the electrophilic substitution of benzene but not for the electrophilic addition of alkene?- alkenes contain an electron-rich C=C bond that can attack electrophiles with a partial positive charge - due to resonance stability of benzene, electrophiles with partial positive charges are too weak to react with benzene- hence, concentrated H2SO4 catalyst reacts with HNO3 to generate NO2+, which is a much stronger electrophile that can react with benzene Why must AlCl3 Lewis acid catalyst be added in anhydrous conditions?- H2O contains LP of electrons on the O atom, so AlCl3 might accept an electron pair from H2O instead of Cl, preventing Cl+ electrophile from forming and electrophilic substitution with benzene cannot occur. Resonance effect vs Inductive effect on substituted benzenes Inductive effect: present when there is a difference in electronegativity between the 2 atoms in a sigma bond.Resonance effect: present when there is an overlap of p orbitals between the C atoms of benzene ring and the substituent bonded to it. Resonance effect > Inductive effect: electrons are delocalised throughout the π electron cloud in resonance effect, while electrons only move within one sigma bond in inductive effect Inductive effect > Resonance effect: For -Cl, -Br, and -I substituent groups, the 3p orbital overlaps less effectively with the 2p orbitals of the C atoms in the benzene ring.

Mechanisms to know: Electrophilic Substitution

PHYSICAL PROPERTIES: simple molecular structure, pd-pd comparison of melting / boiling points between halogenoalkanes with different halogens- iodoalkanes have the largest electron cloud size, most easily polarised- iodoalkanes hence have the strongest id-id, more energy is required to overcome the id-id insoluble in water - the energy released to form pd-pd between halogenoalkanes and water molecules is insufficient to compensate for the energy required to overcome the strong hydrogen bonding between water molecules and pd-pd between halogenoalkane molecules soluble in non-polar organic solvents- the energy released to form id-id between the non-polar organic solvent molecueles and halogenoalkanes is sufficient to compensate for the energy required to overcome the id-id between non-polar organic solvent molecules and pd-pd between halogenoalkanes. Multi-substitution of ammonia: once the amine is formed, further substitution can occur where the lone pair on the N atom of the amine is used to initiate another nucleophilic substitution reaction. This can be prevented by ensuring ammonia is in excess and the reaction is done in a sealed tube. FAQ Why do products of SN1 form a racemic mixture?- The intermediate carbocation is trigonal planar wrt to the electron-deficient carbon, so the nucleophile is able to attack the top and bottom faces of the carbocation with equal probability.- equal amounts of each enantiomer is formed, no net rotation of plane-polarised light, hence a racemic mixture is formed. What factors affect the rate of nucleophilic substitution?1. steric hindrance- as the number of alkyl groups bonded to the C atom in the C-X bond increases, steric hindrance increases, rate of reaction decreases.2. stability of carbocation- as the number of alkyl groups bonded to the C atom in the C-X bond increases, the stability of the carbocation formed in step 1 increases, rate of reaction increases 3. type of halogen atoms in the C-X bond- ease of nucleophilic substitution depends on the ease of breaking the C-X bond- as the bond energy of the C-X bond decreases down the group, halogenoalkanes react more readily. Why are halogenoarenes inert to nucleophilic substitution?1. partial double bond character- p-orbital of the halogen atom overlaps with the π electron cloud system of the benzene ring.- the lone pair of electrons on halogen atoms can be delocalised into the benzene ring, giving the C-X bond partial double bond character, making it stronger and harder t