Organic chemistry

• Functional group

  A group of atoms within a molecule that has a characteristic chemical behaviour
• Homologous series

  A group of molecules of the same general formula, carrying the same functional group and exhibiting similar chemical properties
• Alkanes

  • Undergo substitution reactions
• Homolytic bond breaking

  Symmetrical bond breaking
• Heterolytic bond breaking

  Unsymmetrical bond breaking
• Note the difference in arrows used! One-headed 'fishhook' arrows are used to show homolytic bond breaking while double headed arrows are used to show heterolytic bond breaking
• The opposite pathway is true, bond formation can be symmetrical or unsymmetrical
• Halogenation

  Substitution of a hydrogen atom of an alkane with a halogen

  
• Halogenation

  • Occurs through free radical substitution
  • Relies on the high reactivity of radicals (species carrying an odd number of electrons)
  • Radicals can abstract an atom from a molecule
• Halogenation reaction (using chlorine as an example)

  1. Initiation: Irradiation of the halogen with UV light initiates the homolytic breakage of the covalent bond in some of the halogen molecules. A few halogen radicals are formed.

  2. Propagation: A reactive halogen radical collides with a molecule of the alkane, abstracting a hydrogen atom from it to produce the hydrogen halide and an alkyl radical. The alkyl radical goes on to react with another halogen molecule, releasing a halogen radical and forming a haloalkane. The halogen radical goes on to continue propagation, leading to a chain reaction.

  3. Termination: When two radicals collide, they can form a bond and end a reaction cycle, ending the chain propagation. As the number of radicals at any time is very small, these reactions are infrequent.

  
• Cracking

  Breakdown of saturated hydrocarbon molecules (alkanes) into smaller (usually more useful) products (e.g. a smaller alkane and alkene) through the application of heat, pressure and catalysts

  
• Thermal cracking

  • Utilizes high temperatures (450-750°C) and pressures (up to 70 atm)
  • Operates through free radical mechanisms
• Catalytic cracking

  • Utilizes zeolites (complex aluminosilicates of the form Al2O5Si associated with positive ions) along with lower temperatures (~500°C) and moderately low pressures
  • The mechanism includes ionic intermediates

  
• Note that cracking occurs in a fairly random manner, with only relative proportions of alkenes to alkane products being generally modifiable based on conditions
• Combustion reactions

  Exothermic reactions of hydrocarbons with oxygen to produce carbon dioxide and water (complete combustion) and occasionally carbon monoxide and carbon as well (incomplete combustion)
• Complete combustion reactions

  𝐶𝑥𝐻𝑦 + (𝑥 + 𝑦/4) 𝑂2 → 𝑥𝐶𝑂2 + 𝑦/2 𝐻2𝑂
• Alkenes

  • More reactive than alkanes due to the presence of a carbon-carbon double bond
  • The electrons of the 𝜋 molecular orbital are of a higher energy than those of the 𝜎 molecular orbital
  • Alkanes are less reactive than alkenes because they lack this more reactive bond
• Electrophilic addition reactions

  Reactions where a molecule is added across the double bond, giving a single product
• Reaction of alkene with Br2(l)

  1. The addition of bromine dissolved in an organic solvent (like tetrachloromethane, CCl4) to an alkene (like ethene)

  2. The product is a vicinal dihaloalkane (a dihaloalkane with both halogen substituents on adjacent carbons). Here it is 1,2-dibromoethane.

  3. A dipole is induced in the highly polarizable bromine molecule by the electron rich 𝜋 bond. The partially positive bromine acts as an electrophile, attacking the double bond.

  4. The result is the bromonium ion.

  5. The remaining bromide ion attacks the bromonium ion from the opposite side, compensating for the positive charge.
• Reaction of alkene with Br2(aq)

  1. The result is a bromoalkanol and HBr, where the bromine substituent and the hydroxyl group are on adjacent carbons (across the double bond).

  2. The reaction mechanism is similar to the previous one except that water becomes attracted to the intermediate bromonium instead of the bromide ion.

  3. The OH- from the water combines with the bromonium instead, releasing a proton which combines with the remaining bromide ion.

  
• Reaction of alkene with COLD Acidified Potassium Manganate (VII)

  1. The mixture will turn from purple to colourless, as the electron rich double bond of the alkene acts as a reducing agent.

  2. The product will be a vicinal diol, also called a glycol, which has the hydroxyl groups on adjacent carbons.
• Reaction of alkene with HOT (and concentrated) Acidified Potassium Manganate (VII)


  1. The alkene undergoes extended oxidation, in two main stages.

  2. Stage 1: The carbon-carbon double bond is broken and replaced with two carbon-oxygen double bonds on two separate carbonyl compounds.

  3. Stage 2: Further oxidation - If a product is a ketone (both R groups attached to the carbonyl group are alkyl groups), it will not undergo further oxidation. If a product is an aldehyde (one of the R groups on the carbonyl group is a hydrogen), it will be further oxidized to a carboxylic acid. If a product is formaldehyde/methanal (both R groups are hydrogen), it will be oxidized first to methanoic acid, then to carbon dioxide and water.
• Reaction of alkene with Concentrated Sulphuric Acid (Hydration)


  1. The product of the reaction (when heated) is the alkanol. The acid acts as a catalyst.

  2. The reaction actually proceeds by the addition of the sulphuric acid across the double bond to form an alkyl hydrogensulfate, which then goes on to react with water to produce the alkanol.
• Reaction of alkene with Hydrogen Halides (using HBr as an example)

  1. HBr is a polar molecule, as bromine is far more electronegative and attracts the electrons of the covalent bond far more strongly. The partially positive hydrogen atom acts as an electrophile to initiate the reaction with an electrophilic attack of the electron-rich 𝜋 bond.

  2. Two possible carbocations (positively charged hydrocarbon ions) are formed, one is secondary and the other is primary. The secondary carbocation is more stable and more likely to be formed.

  3. The regioselectivity of this reaction is explained by Markovnikov's Rule, which states that the hydrogen will be added to the less substituted carbon while the halogen goes to the more substituted carbon.
• Reaction of alkene with Hydrogen (Hydrogenation)

  1. The product is an alkane.

  2. The reaction involves an insoluble metal catalyst like palladium (Pd-C), platinum (PtO2) or nickel (Ra-Ni) and occurs at 150°C.

  3. Vegetable oils often contain high proportions of polyunsaturated and mono-unsaturated fats (oils), and tend to be liquids at room temperature. You can "harden" (raise the melting point of) the oil by hydrogenating it in the presence of a nickel catalyst. The relatively high temperatures tend to switch some cis carbon-carbon double bonds into trans double bonds, which will remain in the final product if not hydrogenated. Trans fats are associated with high LDL blood levels.
• Alcohols


  Designated as primary, secondary and tertiary based on the number of carbons to which the carbon carrying the hydroxyl group is joined

  
• Oxidation of alcohols

  Primary alcohols are oxidized to carboxylic acids, secondary alcohols are oxidized to ketones, and tertiary alcohols are not oxidized.
• Esterification

  When heated under reflux with concentrated sulphuric acid and a carboxylic acid, an alcohol forms an ester.
• Dehydration of alcohols

  Occurs at 180°C and produces the alkene and water.
• Iodoform test

  Involves adding iodine solution to a small volume of the sample, followed by just enough sodium hydroxide to decolourise the iodine. A positive result is the formation of a pale yellow precipitate, indicating the presence of an alcohol of the form CH3-CH(OH)-R or a carbonyl compound of the form CH3-CO-R.
• Iodoform test mechanism

  1. Step 1: Formation of NaIO by reaction between I2 and NaOH.

  2. Step 2: (If the sample is an alcohol) Oxidation of the alcohol by NaIO to form a carbonyl compound.

  3. Step 3: Substitution of the hydrogen atoms of the methyl group with iodine atoms.

  4. Step 4: Cleavage of the carbon-carbon bond to release triiodomethane (iodoform) from the rest of the molecule by hydroxide ions.
• Halogenoalkanes/Haloalkanes/Alkyl Halides

  Undergo mainly nucleophilic substitution reactions
• Hydrolysis of halogenoalkanes

  1. Usually alkaline hydrolysis, which requires an alkaline solution like NaOH(aq).

  2. The products are an alcohol and the hydrogen halide and water.

  3. Primary (and usually secondary) haloalkanes undergo a particular mechanism called SN2 (substitution, nucleophilic, bimolecular).

  4. Tertiary (and sometimes secondary) haloalkanes typically undergo a type of reaction called the SN1, where the 1 stands for unimolecular (only one molecule is involved in the rate determining step).

  
• Nucleophilic attack

  1. Hydroxide ion from NaOH attacks

  2. Transition state formed with pentavalent carbon atom

  3. Bromide ion displaced

  
• Bromide ion

  Better leaving group than hydroxide ion due to greater stability
• SN1 reaction

  • Unimolecular (only one molecule involved in rate determining step)
  • Occurs for tertiary (and sometimes secondary) haloalkanes
  • Rate determining step is spontaneous ionization of haloalkane to give carbocation and halide ion
  • Bond making and bond breaking occur in a stepwise manner

  
• Carbonyl group

  Polar group, oxygen atom pulls strongly on electrons in carbon-oxygen double bond
• Carbonyl compounds

  Aldehydes and ketones
• Reaction with NaCN(aq)/H+(aq) (Nucleophilic Addition)


  1. NaCN reacts with H+ to form HCN and Na+

  2. Cyanide ion acts as nucleophile, attacking partial positive charge on carbon of carbonyl

  3. Negatively charged ion formed

  4. Hydrogen from HCN or acid combines with ion to form hydroxynitrile

  
• Reduction of carbonyl compounds

  Addition of a hydrogen atom (electron gained)