Organic Reaction Mechanisms

Formulae, Functional Groups, and Naming of Organic Compounds

• Hydrocarbons consist of only carbon and hydrogen atoms, with alkanes being simple hydrocarbons lacking functional groups.
• Naming organic compounds with up to 6 carbons in a chain is essential in organic chemisty.
• The prefix of the name depends of the number of carbons: meth-(1), eth-(2), prop-(3), but-(4), pent-(5), hex-(6).

Ways to Display Organic Compounds

• Molecular Formula - Indicates the actual number of atoms of each element in a molecule.
• Structural formula - Illustrates the structure, carbon by carbon, including attached hydrogens and functional groups.
• Skeletal Formula - Shows bonds of the carbon skeleton, omitting carbon and hydrogen atoms, but including functional groups for simplifying complex structures.
• Displayed Formula - Shows all atoms and bonds, arranged to show every bond between them.
• A homologous series features similar compounds with the same functional group, the successive compounds vary by -CH2 that dictates the compound's reaction.
• Table 1 describes various Aliphatic compounds including Alkanes, Alkenes, Halogenoalkanes, Alcohols, Aldehydes, Ketones, Carboxylic acids, Esters, Amines and Nitriles, as well as their prefixes, suffixes, example names, molecular formula, structural formula, skeletal formula and displayed formula.

Naming Organic Compounds

• Follow these rules when naming organic compounds:
• Identify the longest carbon chain that contains the functional group, which determines the root name.
• Identify the functional group that dictates the suffix or prefix.
• Number the carbon chain so that the functional group has the lowest possible number.
• Add side chains as prefixes, ordering alphabetically and indicating quantity with prefixes like di-, tri-, tetra-.

Characteristic Organic Reactions Terminology

• Functional Group is a group of atoms is responsible for the charateristic reactions of an organic compound.
• Saturated molecules contain only single carbon-carbon bonds, holding the maximum possible hydrogen atoms.
• Unsaturated molecules contain carbon-carbon double or triple bonds, unable to contain the maximum number of hydrogen atoms.
• Homolytic fission involves the splitting of a bond where each atom keeps one electron.
• Heterolytic fission involves the splitting of a bond where one atom keeps both electrons.
• Free Radical is an uncharged molecule with an unpaired electron.
• Initation is a chemical reaction that triggers further ones.
• Propagation is a reaction with no net gain or loss of free radicals.
• Termination is when a reactive intermediate is rendered inactive.
• Nucleophile donates electron.
• Electrophile accepts electron pair.
• Addition is when two molecules react to form a large molecule.
• Substitution is when an atom/group is replaced.
• Elimination is when too substituents are removed from a molecule.
• Hydrolysis is the splitting up of a compound with water.
• Oxidation is the loss of electrons.
• Reduction is the gain of electrons.

Shapes of Organic Molecules

• Organic compounds are described as straight-chained, branched or cyclic, depending on how the carbon atoms are arranged.

Hybridisation

• Hybridisation is the mixing of atomic orbitals to form new hybrid orbitals suitable for bonding.
• The different types of hybridisation include: sp, sp² and sp³.

Hybridisation of Carbon

• The normal carbon's electron configuration is 1s22s22p².
• If the carbon atom receives the right amount of energy, an electron can be promoted so the new configuration is 1s22s12p³.
• This a favorable process so it is more stable.
• The configuration change from 1s22s22p² to 1s22s12p³ leads to less repulsion and more stability when the electrons are unpaired.
• The hybridisation process itself is favourable thanks to the electrons becoming unpaired.

Types of Hybridisation

• sp hybridisation - Happens when One s orbital and one p obital hybridise to form two equivalent orbitals, which repel to give a linear shape with a 180° bond angle.
• sp² hybridisation - Happens when One s orbital and two p orbitals hybridise to form three equivalent orbitals, where the remaining 2p orbital is left unchanged. The three new sp² orbitals all repel to give a trigonal planar arrangement with a bond angle of 120°. The unchanged 2p orbital lies perpendicular to this planar arrangement.
• sp³ hybridisation - Happens when One s orbital and three p orbitals hybridise to form four equivalent orbitals, molecule has bond angles of 109.5°.
• Bonding in Ethane (sp³) - An sp³ orbital overlaps with another sp³ orbital to form a C-C covalent bond. The other three orbitals overlap with the s orbital of a hydrogen atom to form more covalent bonds. These bonds between each atom are sigma (ơ) bonds. The shape of an ethane molecule (seen on the right) is tetrahedral, with bond angles of 109.5°.
• Rotation can occur around sigma bonds.

Bonding in Ethene (sp²)

• Two sp² orbitals overlap to form a single sigma C-C bond.
• The unhybridized 2p orbitals also overlap with each other to form a second bond called a pi bond.
• Ethene molecules have a trigonal planar shape with bond angles of 120°.
• The molecule is planar because there is restricted rotation around the C=C pi (π) bond.
• The bond is formed when orbitals overlap below and above the plane of atoms.

Isomerism

• Isomers are molecules with the same molecular formula but different arrangement of atoms.

Structural Isomers:

• Chain, position and functional group.
• Chain isomerism: occurs when a different arrangement of the same molecule, there is branching on the carbon chain.
• Functional group isomerism: occurs when the functional groups on the carbon chain changes.
• Position isomerism: this occurs when The carbon chain backbone remains the same but attached groups change position.

Stereoisomerism

• Stereoisomers are molecules with the same molecular and structural formula with a different spatial arrangement of atoms.
• Geometrical (cis-trans) isomerism and optical isomerism are both types of stereoisomerism.

Geometrical (cis-trans) Isomerism

• Also called E/Z isomerism, originates from restricted pir bond rotation around a C=C double bond.
• Due to lack of rotation, attached groups’ position is fixed, allowing different isomers.
• The 'cis' isomer has highest priority groups on the same end, and the 'trans' isomer has them diagonally across.
• The priority of a group is decided by atomic number.

Optical Isomerism

• It occurs because of a chiral centre wich is a center atom with four different groups bonded to it resulting in non-superimposable mirror images.
• If a compound has a chiral centre, it will display optical isomerism and is possible to have more than one chiral.

Questions cover curly arrows, free-radical substitution, electrophilic addition, and homologous series. Explore the role of curly arrows, steps in free-radical mechanisms, and characteristics of homologous series. Test your knowledge of organic chemistry concepts.