Organic Chemistry Notes

Carbon Chemistry (IB Unit 10)

Carbon Chemistry and Catenation

Carbon's unique ability to form covalent bonds with itself, known as catenation, allows it to create long chains and rings. This leads to a vast number of carbon-containing compounds, literally millions, known to exist. Carbon can form four bonds, allowing for diverse structures.

Carbon Bonding and Structures

• Carbon can form four bonds, leading to long chains.

• Alkanes have only hydrogen atoms attached to the side positions on the chains.

• Other atoms can attach to form different organic compound families.

• Double bonds can occur in simple molecules and long chains.

• Carbon atoms can form ring molecules, such as glucose.

• Long-chain fat molecules and numerous other molecules can be formed.

  Here is an illustration using LaTex of possible molecular structures:

  $\text{Carbon} \bigcirc$
  $\text{Hydrogen} \text{●}$
  $\text{Oxygen}$

Homologous Series

Classification of carbon compounds into families or series is essential due to their vast number. A homologous series is a series of closely related compounds.

Alkanes

The first homologous series is the alkanes, which are hydrocarbons composed of only carbon and hydrogen atoms. They are good fuels because they react with oxygen in exothermic reactions, producing carbon dioxide and water.

Each successive member of the alkane series differs by a $CH<em>2$ group. For example, ethane ($C</em>2H<em>6$) is followed by propane ($C</em>3H<em>8$). Members of the same series have the same general formula and functional group. For alkanes, the general formula is $C</em>nH_{2n+2}$. The functional group is the alkyl group, containing carbon-carbon single bonds.

Alkane Properties

Boiling Point Trend in n-Alkanes

The boiling point increases with the number of carbon atoms due to stronger London dispersion forces.

Molecular and Structural Formulae

Organic compounds can be represented by empirical, molecular, and structural formulae, each providing different information. Structural formulae include full, condensed, and skeletal forms.

Empirical and Molecular Formulae

The empirical formula represents the simplest whole-number ratio of atoms in a compound.

• For methane and propane, the empirical formula is the same as the molecular formula.

• For ethane ($C<em>2H</em>6$), the empirical formula is $CH_3$.

The molecular formula represents the actual number of each type of atom in a compound.

• Butane: $C<em>4H</em>{10}$

• Ethanol: $C<em>2H</em>5OH$

• Ethanoic acid: $CH<em>3COOH$ or $CH</em>3CO_2H$

Structural Formulae

• Full Structural Formulae: Shows all atoms and bonds.

• Condensed Structural Formulae: Omits bonds between atoms (e.g., propane as $CH<em>3CH</em>2CH_3$).

• Skeletal Formulae: Omits all atoms except for functional groups, with carbon atoms at line intersections and ends. Hydrogen atoms bonded to carbon are not shown (e.g., propan-1-ol and but-2-ene).

3D Representations

Representing organic molecules in 3D uses stereochemical formulae, showing the relative positions of atoms or groups around a carbon atom. Solid wedges indicate bonds sticking out towards the viewer, and broken lines indicate bonds directed away from the viewer.

Branched and Cyclic Structures

Alkanes can have straight-chain or branched-chain structures (e.g., 2,2,4-trimethylpentane). Carbon atoms can also join in rings, forming cyclic alkanes, often represented by polygons.

Structural Isomerism

Isomers have the same molecular formula but different arrangements of atoms.

Chain Isomerism

Different arrangements of the carbon chain. Compounds have similar chemical properties but different physical properties, such as melting and boiling points (e.g., isomers of $C<em>5H</em>{12}$).

Position Isomerism

Occurs when the functional group is attached in different positions (e.g., butene isomers $C<em>4H</em>8$).

Functional Group Isomerism

Isomers have the same molecular formula but different functional groups (e.g., propanal and propanone, both with the formula $C<em>3H</em>6O$).

Functional Groups

Compounds belonging to a homologous series have the same functional group.

Acidic and Basic Functional Groups

Carboxyl groups act as Brønsted-Lowry acids by donating a proton. Amine groups act as Brønsted-Lowry bases by accepting a proton.

IUPAC Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) system is used for naming organic compounds, using a stem, suffix, and prefix.

1. Stem: Depends on the number of carbon atoms in the longest carbon chain (meth-, eth-, prop-, but-, pent-, hex-).

   Number of Carbon Atoms	Stem
   1	meth-
   2	eth-
   3	prop-
   4	but-
   5	pent-
   6	hex-

2. Suffix: Differs according to the functional group present.

3. Prefix: Identifies the position of substituent groups.

Naming Straight-Chain Alkanes

Count the number of carbon atoms in the longest chain and add the suffix -ane (e.g., methane, ethane, propane, butane).

Naming Branched-Chain Alkanes

1. Identify the longest continuous carbon chain (stem).

2. Identify and name side-chains (prefix); use di-, tri-, tetra- prefixes for multiple side-chains of the same type. List different side-chains in alphabetical order.

3. Identify the position of side-chains using numbers, numbering the carbon atoms to give the lowest number to the substituents.

Example: 2,3-dimethylpentane and 3-ethyl-4-methylhexane.

Naming Alkenes

Use the stems with the suffix -ene. Indicate the position of the carbon-carbon double bond with a number (lowest possible).

Naming Alkynes

Use the stems with the suffix -yne. Indicate the position of the carbon-carbon triple bond with a number (lowest possible).

Naming Alcohols

Use the stems with the suffix -anol. Use a number to indicate the position of the hydroxyl group.

Diols (containing two hydroxyl groups) are named by identifying the longest chain and the position of the hydroxyl groups.

Naming Carboxylic Acids

Use the stems with the suffix -anoic acid (e.g., methanoic acid, ethanoic acid).

Naming Aldehydes and Ketones

Aldehydes use the stems with the suffix -anal (e.g., methanal).
Ketones use the stems with the suffix -anone. Use numbers to indicate the position of the carbonyl group.

Naming Ethers

The shorter alkyl group is given the prefix alkoxy- (e.g., methoxy-, ethoxy-), and the longer alkyl group is named as an alkane.

Naming Nitriles

Add the suffix -nitrile, with the carbon atom of the nitrile group as the first atom of the chain.

Naming Amines

Primary amines: use stems + suffix -amine. Secondary and tertiary amines are named based on the longest unbranched carbon chain bonded to the nitrogen atom, with prefixes like methyl-, ethyl-, propyl- with an italicised N.

Naming Amides

Replace the -oic suffix of the carboxylic acid with -amide. Secondary and tertiary amides are named similarly to the amines.

Trends in Physical Properties

The physical properties of organic compounds depend on intermolecular forces: London dispersion forces, dipole-dipole attractions, and hydrogen bonding. Hydrogen bonding is the strongest.

Factors Affecting Volatility

1. Molar Mass: Boiling point increases with increasing molar mass (stronger London dispersion forces).

2. Branching: Branched-chain isomers have lower boiling points due to reduced surface contact.

3. Functional Group: Polar functional groups result in stronger dipole-dipole interactions and higher boiling points. Compounds with O-H or N-H bonds can form hydrogen bonds, increasing boiling points.

Comparison requires similar molar masses. For example, ethanol (C2H5OH) has a higher boiling point than propane (C3H8) due to hydrogen bonding. Carboxylic acids can form dimers through hydrogen bonds, increasing boiling points.

Solubility of Organic Compounds

Solubility in water depends on the ability to form hydrogen bonds. Methanol, ethanol, and propanol are completely soluble. Solubility decreases as the hydrocarbon chain length increases due to increased hydrophobic character. Non-polar molecules have limited solubility in water.

Classification of Organic Compounds

Alcohols and Halogenoalkanes

Primary, secondary, and tertiary classifications refer to the number of carbon atoms bonded to the carbon atom directly bonded to the functional group.

Amines

Classified as primary, secondary, or tertiary based on the number of alkyl groups bonded to the nitrogen atom.

Aromatic Hydrocarbons

Aromatic compounds, or arenes, contain a benzene ring (phenyl functional group). Benzene ($C<em>6H</em>6$) is the simplest arene.

Evidence Against Kekulé's Structure

1. All carbon-to-carbon bond lengths in benzene are identical (140 pm), intermediate between single (154 pm) and double (134 pm) bonds.

2. Only one isomer exists for 1,2-disubstituted benzene compounds.

3. Benzene undergoes electrophilic substitution reactions rather than addition reactions.

4. The enthalpy change of hydrogenation of benzene is lower than expected, indicating greater stability due to resonance energy (-152 kJ mol-1).

Structure of Benzene

Each carbon atom is bonded to two other carbon atoms and one hydrogen atom, with delocalised electrons shared between more than two nuclei. Delocalised electrons form two 'donut-shaped' rings above and below the benzene molecule. The delocalised electrons are represented by a circle in the middle of the hexagon structure, known as a pi electron cloud.

Functional Group Chemistry

Importance of Hydrocarbons

The fractional distillation of crude oil separates it into useful fractions for fuels and chemical feedstock. Cracking converts less commercially viable fractions into more useful hydrocarbons like ethene.

Natural Gas

Natural gas is an important fossil fuel; its major component is methane. Methane is a greenhouse gas released from various sources. Methane clathrates are methane molecules enclosed in ice structures, found in sediments beneath the deep oceans, Antarctic ice, and Arctic permafrost. Releasing methane from these deposits poses environmental concerns due to its global warming potential.

Alkanes

Alkanes are saturated hydrocarbons with carbon-carbon single bonds. They are unreactive due to non-polar carbon-hydrogen bonds and strong covalent bonds, making them kinetically stable.

Combustion Reactions of Alkanes

Alkanes burn in oxygen in exothermic reactions. Complete combustion produces carbon dioxide and water. Incomplete combustion produces carbon monoxide and water, or solid carbon and water. Carbon monoxide is a poisonous gas.

• Complete Combustion:
  $CH<em>4 (g) + 2O</em>2 (g) \rightarrow CO<em>2 (g) + 2H</em>2O (l)$
  $C<em>3H</em>8 (g) + 5O<em>2 (g) \rightarrow 3CO</em>2 (g) + 4H_2O (l)$

• Incomplete Combustion:
  $2C<em>3H</em>8 (g) + 7O<em>2 (g) \rightarrow 6CO (g) + 8H</em>2O (l)$
  $C<em>3H</em>8 (g) + 2O<em>2 (g) \rightarrow 3C (s) + 4H</em>2O (l)$

Free-Radical Substitution Reactions

Alkanes undergo free-radical substitution reactions with halogens (e.g., chlorine). These photochemical reactions require UV radiation. The reaction proceeds in three steps: initiation, propagation, and termination.

• Example:
  $CH<em>4 (g) + Cl</em>2 (g) \rightarrow CH_3Cl (l) + HCl (g)$
  (conditions: UV radiation)

1. Initiation: The bond between halogen atoms is broken by UV radiation, forming free radicals.

2. Propagation: A chlorine free radical reacts with methane, producing a methyl radical and hydrogen chloride. The methyl radical reacts with a chlorine molecule, producing chloromethane and another chlorine radical.

3. Termination: Two free radicals recombine, removing free radicals from the reaction mixture.

Alkenes

Alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond. They have a trigonal planar arrangement around the double bond (120° bond angle). They are more reactive than alkanes and undergo addition reactions.

Addition Reactions of Alkenes

1. Hydrogenation: Addition of gaseous hydrogen across the carbon-carbon double bond to produce an alkane (high pressure, temperature, nickel catalyst).

   Used in the margarine industry to convert unsaturated vegetable oils into saturated compounds.

2. Hydration: Reaction with steam in the presence of a sulfuric acid or phosphoric acid catalyst to produce an alcohol.

   Used in industry to produce alcohols like ethanol and butan-2-ol from alkenes.

3. Halogenation: Reaction with halogens to produce dihalogenoalkanes (readily at room temperature).

   Used as a test for unsaturation—bromine decolourises when added to an alkene.
   Alkenes also react with hydrogen halides to produce halogenoalkanes.
   $C<em>2H</em>4 (g) + HBr (g) \rightarrow C<em>2H</em>5Br (g)$
   Similar reactions occur with hydrogen chloride and hydrogen iodide.

Addition Polymerisation

Alkene molecules (monomers) bond to form long chains (polymers). Polymers contain thousands of monomer units. Poly(ethene) (polythene) was the first poly(alkene) synthesised.

• General Equation: $n(monomer) \rightarrow (monomer)_n$
  Chains formed are saturated, but the polymer name retains the -ene suffix (e.g., poly(propene)).

Examples:

• Poly(chloroethene) (PVC): Used for construction materials and electrical cable covers, but synthesis produces toxic dioxins.

• Poly(tetrafluoroethene) (PTFE): Used for non-adhesive surfaces (e.g., non-stick pans, Gore-tex®).

Properties and Uses of Polymers:

Reaction Map

$Alkene + H<em>2O \rightarrow^ {(conc.H</em>2SO<em>4)} Alcohol$ $Alkene + H</em>2\rightarrow^ {(Ni \ catalyst, 180°C)} Alkane$
$Alkene + HBr \rightarrow Halogenoalkane$

Alcohols

The physical and chemical properties of alcohols are determined by the hydroxyl (-OH) functional group. Alcohols can form hydrogen bonds with other alcohol molecules and with water molecules, increasing solubility.

Reactions of Alcohols

1. Combustion Reactions: Alcohols burn in oxygen in exothermic reactions. Complete combustion produces carbon dioxide and water. If oxygen is limited, carbon monoxide and water or carbon and water are produced.

   $2CH<em>3OH (l) + 3O</em>2 (g) \rightarrow 2CO<em>2 (g) + 4H</em>2O (l)$
   $\Delta H\degree<em>c = -726 \ kJ \ mol^{-1}$ $C</em>2H<em>5OH (l) + 3O</em>2 (g) \rightarrow 2CO<em>2 (g) + 3H</em>2O (l)$
   $\Delta H\degree_c = -1367 \ kJ \ mol^{-1}$
   Bioethanol is made from plant material and is used in Brazil to reduce reliance on fossil fuels.

2. Oxidation Reactions: Alcohols are susceptible to oxidation in the presence of oxidising agents like potassium manganate(VII) and acidified potassium dichromate(VI).

   Primary alcohols produce aldehydes or carboxylic acids, depending on the degree of oxidation. Secondary alcohols produce ketones. Tertiary alcohols do not undergo oxidation.

   Oxidation of Primary Alcohols: Partial oxidation yields an aldehyde (removed by distillation). Complete oxidation yields a carboxylic acid (heat under reflux).

   $Primary Alcohol \rightarrow^ {[O]} Aldehyde \rightarrow^ {[O]} Carboxylic Acid$

   Oxidation of Secondary Alcohols: Yields a ketone (heat under reflux).

   $Secondary Alcohol \rightarrow^ {[O]} Ketone$

Example Boiling Points:

3. Esterification: Reactions between alcohols and carboxylic acids to form esters (condensation reactions), with the loss of water and a strong acid catalyst. Esters are named from the alcohol and carboxylic acid (e.g., ethyl ethanoate).

   $Alcohol + Carboxylic Acid \rightleftharpoons^ {(H<em>2SO</em>4\ catalyst)} Ester + Water$

Esters are volatile with distinctive aromas and are used in synthetic flavours and perfumes.

Halogenoalkanes

Halogenoalkanes have one or more hydrogen atoms replaced by a halogen atom (general formula $C<em>nH</em>{2n+1}X$). They are oily liquids that do not mix with water. They undergo nucleophilic substitution reactions, where an atom or group is replaced by another atom or group.

Nucleophilic Substitution Reactions

The carbon-to-halogen bond is polar, making the carbon atom electron-deficient and open to attack by nucleophiles (e.g., hydroxide ion $OH^−$).

• Example:

  $CH<em>3CH</em>2Cl (l) + NaOH (aq) \rightarrow CH<em>3CH</em>2OH (l) + NaCl (aq)$
  The reaction involves the hydroxide ion $OH^−$ from a strong alkali (e.g., sodium hydroxide) as the nucleophile. Reaction conditions are heat with a dilute solution of sodium hydroxide or potassium hydroxide.

Benzene Reactions

Electrophilic Substitution

Benzene’s stability is due to delocalised electrons. It undergoes substitution reactions to maintain stability. Delocalised electrons attract electrophiles (positive charge), leading to electrophilic substitution reactions.

• Example:

  $C<em>6H</em>6 + NO<em>2^+ \rightarrow C</em>6H<em>5NO</em>2$
  (nitronium ion forms nitrobenzene, catalyst: concentrated sulfuric acid ($H<em>2SO</em>4$))

  Benzene also undergoes electrophilic substitution with halogens (e.g., chlorine with aluminium chloride ($AlCl_3$) catalyst).

Combustion

Benzene, like all hydrocarbons, can undergo combustion to form carbon dioxide and water.

$2C<em>6H</em>6 (l) + 15O<em>2 (g) \rightarrow 12CO</em>2 (g) + 6H_2O (l)$