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AS Level Chemistry 9701
13. An introduction to AS Level organic chemistry
Written by: Pranav I
Formatted by: Pranav I
Index
13.1 Representing organic molecules
- The empirical formula represents the simplest ratio of the different types of atoms present in the molecule
- The molecular formula represents the actual numbers of each type of atom in a molecule
- The structural formula shows how many, and the symbols of, atoms bonded to each carbon atom in an organic molecule (highlights functional groups)
- Side chains are shown in brackets
- The displayed formula is a 2D representation of an organic molecule, showing all its atoms and their bonds
- The skeletal formula is a simplified displayed formula with all C and H atoms and C-H bonds removed (carbon to carbon bonds are left in place)
- All other atoms and their respective bonds are shown in this representation
- Structural, 3D, and skeletal formulae are usually combined while representing molecules
13.2 Homologous series of organic compounds
- Homologous series: A group of organic compounds having the same functional group, the same general formula, and similar chemical properties
- Functional group: An atom or group of atoms in an organic molecule that determines its characteristic chemical reactions
- General formula: A formula that represents a homologous series of compounds using letters and numbers (molecular formulae attained by substitution)
13.3 Naming organic compounds
- The stem of each name indicates the number of carbon atoms in the longest chain in one molecule of the compound
| Number of carbon atoms | Stem used in naming |
|---|---|
| 1 | Meth- |
| 2 | Eth- |
| 3 | Prop- |
| 4 | But- |
| 5 | Pent- |
| 6 | Hex- |
| 7 | Hept- |
| 8 | Oct- |
| 9 | Non- |
| 10 | Dec- |
- Position of side chains or functional groups indicated by numbering carbon atoms in the longest chain
- Start at the end that produces the lowest possible numbers in the name (check the sum when more than one side chain are present)
- More of the same side chain or functional group is indicated by di-, tri-, or tetra-
- Commas separate adjacent numbers
- A hyphen separates numbers and words
- More than one type of alkyl side-chain → listed in the name in alphabetical order
13.4 Bonding in organic molecules
- Organic molecules can either be straight-chained, branched or cyclic
Sigma (σ) bonds
- A carbon atom has six electrons (1s22s22p2)
- Can form four single covalent bonds with other atoms to gain noble gas electronic configuration (Ne)
- In such cases, a carbon atom has four σ bonds
- Form a tetrahedral structure (bond angle: 109.5°) as bond pairs repel each other
Pi (π) bonds
- Carbon atoms also form double and triple bonds
- C=C double bond
- Consists of a σ bond and a π bond
- Each C atom involved will form three σ bonds and a π bond (sp2 hybridization)
- One spare outer electron in the 2p orbital of each of the C atoms
- The two p orbitals overlap to form a π bond
- Two lobes of the π bond lie above and below the plane of the atoms in the molecule to maximize overlap of the p orbitals
- Three bond pairs in the σ bonds in the plane → bond angle is 120° (planar)
- The π bond pair is not considered since it is in a different plane
- C=C triple bond
- Made up of a σ bond and two π bonds
- Each C atom involved will form two σ bonds and two π bonds (sp hybridization)
- Two spare outer electrons in the 2p orbital of each of the C atoms
- The two p orbitals overlap to form two π bonds
- Two lobes of one π bond lie above and below the plane of the atoms while the two lobes of the other π bond lie in front of and behind the plane of the atoms in the ethyne molecule to maximise overlap of the p orbitals
- Two bonds pairs in the σ bonds in the plane → bond angle is 180° (linear)
13.5 Structural isomerism
✅ Definition
Structural isomers are compounds with the same molecular formula but different structural formulae
Positional isomerism
- The location of the functional group varies in each isomer
- Take care not to repeat the same structure
Functional group isomerism
- Different functional groups present
- They have different chemical properties
- Possible combinations:
- Ketone and aldehyde (e.g. propanone and propanal)
- Ester and carboxylic acid (e.g. methyl methanoate and ethanoic acid)
- Alkene and cycloalkane (e.g. butene and cyclobutane)
Chain isomerism
- The structure of the compound’s carbon skeleton differs
13.6 Stereoisomerism
✅ Definition
Stereoisomers are compounds with the same structural formula but different arrangements of atoms in space
Geometrical (cis-/trans) isomerism
- Restricted free rotation about a C=C bond due to the presence of a π bond
- Cis means same and trans means across
- They have different physical properties and show some variation in chemical properties
- If one of the double-bonded C atoms has the same atoms or groups attached to it, then the compound cannot exhibit geometric isomerism
- Possible in substituted cyclic compounds due to the restricted free rotation about the C-C bonds in the ring
- In the cis-isomer, the two substituted groups are either both pointing above or below the ring
- In the trans-isomer, the two substituted groups are pointing in opposite directions
Optical isomerism
✅ Chiral center: a carbon atom with four different atoms or groups attached to it
- Condition to form optical isomers → contain at least one chiral center
- Optical isomers differ in their effect on polarised light
- Rotate the plane of polarised light in equal amounts but in opposite directions (clockwise and anticlockwise)
Number of optical isomers for a molecule = 2n, where n is the number of chiral centers
- Substituted cyclic compounds can also have chiral centers in their ring structures
- Symmetrical substituted compounds will not have a chiral center
🚨 The Mercedes method to identify the number of chiral centers in a substituted cyclic compound
- In a given skeletal formula, the areas with a ‘Mercedes logo’ contain carbon atoms bonded to four atoms or groups
- Only count the logos with carbons bonded to different groups
- Do not simply count every such structure
- Carbon 1 is a chiral center since it has the Mercedes logo with an -OH group (4 different groups)
- Carbon 2 seems to have the Mercedes logo, but it has a double bond → NOT CHIRAL
- Carbon 3 has the Mercedes logo and has all single bonds, however it is bonded to 2 methyl groups → NOT CHIRAL
13.7 Types of organic reactions
- Addition reactions: An organic reaction in which two or more molecules combine to give a single product molecule
- Elimination reactions: A reaction in which a small molecule is removed from an organic molecule
- Condensation reactions: A reaction in which two organic molecules join together and in the process eliminate a small molecule
- Substitution reactions: A reaction that involves the replacement of one atom, or group of atoms, by another
- Hydrolysis: A reaction in which a molecule is broken down by water (usually sped up by the addition of an acid or an alkali)
- Oxidation reactions (in terms of organic chemistry): A reaction in which oxygen atoms are added to a molecule and/or hydrogen atoms are removed from a molecule
- Reduction reactions (in terms of organic chemistry): A reaction in which oxygen atoms are removed from a molecule and/or hydrogen atoms are added to a molecule
13.8 Types of organic reaction mechanisms
✅ Definition
A reaction mechanism is a series of steps that take place in the course of the overall reaction
- Free-radical substitution
- Electrophilic addition
- Nucleophilic substitution
- Nucleophilic addition
(All of the above mechanisms will be explained in detail in the forthcoming chapters)
- A covalent bond can break in two ways
- Homolytic fission
- Heterolytic fission
Homolytic fission
- Atoms at each end of the bond leave with one electron from the bond pair
- Free radicals are produced → have an unpaired electron and are very reactive
Heterolytic fission
- ‘Uneven’ breaking of a covalent bond
- The more electronegative atom takes both electrons in the covalent bond
