Comprehensive Study Notes on Hydrocarbons: Alkanes and Alkenes

Introduction and Overview of Alkanes

• Alkanes are categorized as SATURATED hydrocarbons, which means they contain the MAXIMUM number of bonded hydrogen atoms possible for the number of carbons present.

• These molecules contain only C-C and C-H SINGLE BONDS.

• General Types of Alkanes:

  • Open chain alkanes: These follow the general formula $C_nH_{2n+2}$. They can exist as straight-chain structures or as branched-chain structures.

  • Cycloalkanes: These follow the general formula $C_nH_{2n}$. These are alkanes formed in the shape of rings.

• Sources: The primary industrial sources of alkanes are natural gas and crude oil.

IUPAC Nomenclature of Straight-Chain Alkanes

1. Count the LONGEST carbon chain in the molecule to determine the parent name.

2. Assign the Prefix: Apply the appropriate prefix based on the number of carbons and add the suffix "-ane".

   • $1$ Carbon: Meth-

   • $2$ Carbons: Eth-

   • $3$ Carbons: Prop-

   • $4$ Carbons: But-

   • $5$ Carbons: Pent-

   • $6$ Carbons: Hex-

   • $7$ Carbons: Hept-

   • $8$ Carbons: Oct-

   • $9$ Carbons: Non-

   • $10$ Carbons: Dec-

IUPAC Nomenclature of Branched-Chain Alkanes

1. Main Carbon Chain: Identify the LONGEST, CONTINUOUS carbon chain. This constitutes the parent name. This chain may not always be straight and might require "turning corners."

2. Numbering: Number the carbons in the main chain starting from the end that results in the LOWEST possible number for the substituents.

3. Substituents: These are groups that substitute for hydrogen atoms on the main chain, such as alkyl groups (e.g., methyl, ethyl, propyl).

4. Writing the Name:

   • Hyphen (-): Used to separate numbers from words.

   • Comma (,): Used to separate numbers from other numbers.

   • Alphabetical Order: If multiple different substituents exist, list them alphabetically (e.g., ethyl appears before methyl).

   • Identical Substituents: Use prefixes such as di-, tri-, tetra-, penta-, or hexa- if the same substituent appears more than once. These prefixes are ignored when determining alphabetical order.

   • Repeated Numbers: Numbers must be repeated for every instance of an identical substituent (e.g., 2,2-dimethylbutane).

   • Selection Criteria: If multiple chains of the same length exist, choose the longest carbon chain that contains the highest number of alkyl groups.

IUPAC Nomenclature of Cycloalkanes

1. Number the carbons in the ring to give the LOWEST possible numbers to the substituents.

2. If there is more than one substituent, utilize prefixes (di-, tri-, etc.).

3. Different substituents must be listed in alphabetical order. For example, in 1-ethyl-2-methylcyclohexane, "Ethyl" is given the number 1 because it is alphabetically prior to "methyl."

4. The name is constructed by adding the prefix "cyclo-" before the alkane name according to the carbon count, ending with the suffix "-ane".

Physical Properties of Alkanes

• Physical State at Room Temperature:

  • $C_1$ to $C_4$: Gases.

  • $C_5$ to $C_{10}$: Liquids.

• Solubility:

  • Insoluble in water: This is because alkanes are non-polar, whereas water is a polar solvent. Alkanes are unable to form hydrogen bonds with water.

  • Soluble in organic solvents: Organic solvents are generally non-polar, making them compatible with alkanes.

• Boiling Points:

  • Boiling point INCREASES as relative MOLECULAR MASS INCREASES. This occurs due to the increase in the strength of Van der Waals forces.

  • Isomer Comparison: Straight-chain alkanes have higher boiling points compared to their branched isomers. This is due to the increase in surface area in straight chains, which strengthens intermolecular forces.

Chemical Properties of Alkanes

• Alkanes are generally UNREACTIVE compounds.

• Reasons for Unreactivity:

  • They possess strong C-C and C-H bonds.

  • The C-C and C-H bonds are non-polar.

  • Consequently, they are NOT ATTRACTED to nucleophiles or electrophiles.

• Reactivity: Despite general stability, they can react with free radicals and undergo combustion with oxygen.

Chemical Reactions of Alkanes

1. Combustion

• Combustion is an EXOTHERMIC reaction; alkanes burn when ignited.

• Complete Combustion: Produces a blue flame.

  • $CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$

• Incomplete Combustion: Produces a yellow flame.

  • $CH_4 + \frac{3}{2} O_2 \rightarrow CO + 2H_2O$

• Soot Production: The amount of soot produced increases as the number of carbon atoms in the alkane increases.

2. Substitution with Halogens (Free-Radical Substitution)

• Reagents: Chlorine ($Cl_2$) or Bromine ($Br_2$) dissolved in Carbon Tetrachloride ($CCl_4$), which acts as a non-polar solvent.

• Condition: Ultraviolet (UV) light is required.

• Products: Halogenoalkanes.

• Observations:

  • Bromine: Changes from reddish-brown to colorless.

  • Chlorine: Changes from pale green to colorless.

• Mechanism: Free-Radical Substitution:

  • Initiation: UV light causes the homolytic fission of the $Cl-Cl$ bond, producing two chlorine free radicals.

  • Propagation Step 1: A chlorine free radical attacks the alkane (e.g., $CH_4$) and breaks a $C-H$ bond. The hydrogen forms a bond with the chlorine, leaving a methyl free radical ($\cdot CH_3$).

  • Propagation Step 2: The methyl free radical attacks a chlorine molecule, breaking the $Cl-Cl$ bond. One chlorine atom bonds with the methyl radical to form chloromethane ($CH_3Cl$), and a new chlorine free radical is generated.

  • Chain Reaction: Steps 1 and 2 repeat in a cycle.

  • Termination: The chain reaction terminates when any two free radicals combine (e.g., two chlorine radicals, two methyl radicals, or one of each).

• Yield Control:

  • This mechanism is not ideal for preparing specific halogenoalkanes as it produces mixtures.

  • Excess Alkane: Results in a high yield of the MONOSUBSTITUTED alkane.

  • Excess Halogen: Leads to further substitution and a mixture of poly-substituted products.

3. Cracking of Alkanes

• Definition: The process of BREAKING LARGE ALKANES into SMALLER ALKANES and ALKENES by breaking C-C bonds.

• This is a random process producing various useful products for the petrochemical industry.

• Types of Cracking:

  • Thermal Cracking: Requires high temperature (approximately $800\,^\circ C$).

  • Catalytic Cracking: Occurs at $450\,^\circ C$ using a catalyst composed of Aluminum(III) oxide ($Al_2O_3$) mixed with silica ($SiO_2$).

• Example Equation:

  • $C_{10}H_{22} \rightarrow C_6H_{14} + C_4H_8$

  • $CH_3(CH_2)_8CH_3 \rightarrow CH_3(CH_2)_4CH_3 + CH_2=CHCH_2CH_3$

Alkenes: Structure and Nomenclature

• Alkenes are UNSATURATED hydrocarbons.

• Functional Group: The Carbon-to-Carbon double bond ($C=C$).

• General Formulas:

  • Open chain alkenes: $C_nH_{2n}$ (These are isomeric with cycloalkanes).

  • Cycloalkenes: $C_nH_{2n-2}$.

• Nomenclature Rules:

  1. Find the longest continuous carbon chain that CONTAINS the $C=C$ bond.

  2. Number the chain from the end closer to the $C=C$ bond to give the double bond the LOWEST possible number.

  3. For chains with more than three carbons, the position of the $C=C$ bond MUST be indicated by a number (e.g., but-2-ene).

  4. In cycloalkenes, the double bond is always at carbons $C_1$ and $C_2$. Number the ring to give substituents the lowest possible numbers, listing them alphabetically.

  5. Example: In 1-ethyl-2-methylcyclohexene, ethyl is given the lowest number due to alphabetical priority.

Physical and Chemical Properties of Alkenes

• Physical Properties:

  • Density: LOWER than that of water.

  • Solubility: Insoluble in water (non-polar); soluble in organic solvents (non-polar).

  • Boiling Point: Increases with relative molecular mass due to increasing Van der Waals forces. Straight chains have higher boiling points than branched isomers.

• Chemical Properties:

  • Most reactions are additions across the $C=C$ bond, which involves breaking the weaker pi ($\pi$) bond.

  • The $C=C$ bond has high electron density, causing it to behave as a NUCLEOPHILE (electron-rich species and electron pair donor).

  • It is readily attacked by electrophiles.

Chemical Reactions of Alkenes

1. Combustion

• Produces $CO_2$ and water.

• Alkenes produce MORE SOOT than alkanes upon combustion.

• Example: $C_2H_4 + 3O_2 \rightarrow 2CO_2 + 2H_2O$

2. Addition of Hydrogen (Hydrogenation)

• Reagents: Hydrogen gas ($H_2$).

• Condition: Nickel ($Ni$) catalyst, heated at $140\,^\circ C$ to $200\,^\circ C$.

• Product: Alkanes.

• Application: Hydrogenation of vegetable oil is used to produce margarine.

3. Addition of Halogens ($X_2$) in Non-Polar Solvent

• Reagents: $Cl_2$ or $Br_2$ in Carbon Tetrachloride ($CCl_4$).

• Condition: Room temperature.

• Product: Halogenoalkane (specifically a dihalogenoalkane).

• Observations: Pale green $Cl_2$ or reddish-brown $Br_2$ turns colorless.

4. Addition with Bromine Water ($Br_2(aq)$)

• Reagent: Bromine water ($Br_2(aq)$).

• Condition: Room temperature, in the dark.

• Product: Haloalcohol (Halohydrin).

• Observation: Reddish-brown solution turns colorless.

• Significance: This is a CONFIRMATION TEST for alkenes. Alkenes decolorize bromine water without light, whereas alkanes require UV light to react and will remain reddish-brown in the dark.

• Regioselectivity: For alkenes with three or more carbons, the Bromine is added to the carbon with the most hydrogens (due to electronegativity differences between $Br$ and $O$, though standard electrophilic addition rules apply to the intermediate).

5. Addition of Hydrogen Halides ($HX$)

• Reagents: $HCl$, $HBr$, or $HI$.

• Condition: Room temperature.

• Product: Halogenoalkane.

• Rule for Unsymmetrical Alkenes: Markovnikov’s Rule states that when $HX$ is added to an unsymmetrical alkene, the HYDROGEN ATOM IS ADDED TO THE CARBON THAT IS BONDED TO THE GREATEST NUMBER OF HYDROGEN ATOMS.

• Mechanism: Electrophilic Addition:

  • Step 1: The $\pi$ bond breaks. Hydrogen (the electrophile) is added to one carbon, creating a carbocation on the other carbon. The $H-X$ bond breaks heterolytically, taking both electrons to $X^-$.

  • Step 2: The nucleophile ($X^-$) attacks the carbocation to form the final addition product.

• Carbocation Stability: The reaction follows the stability of the intermediate carbocation: $3^\circ$ (tertiary) > 2^\circ (secondary) > 1^\circ (primary). Stable carbocations persist longer and form faster.

• Anti-Markovnikov Addition: Occurs ONLY when $HBr$ is used in the presence of PEROXIDES. In this case, the hydrogen attaches to the carbon with FEWER hydrogen atoms. This follows a free radical mechanism.

6. Addition of Steam (Hydration)

• Reagent: Steam ($H_2O(g)$).

• Condition: Phosphoric acid ($H_3PO_4$) catalyst, $300\,^\circ C$, $70\,atm$ pressure.

• Product: Alcohol.

• Rule: Follows Markovnikov’s rule.

7. Mild Oxidation

• Reagent: Dilute acidified Potassium Manganate(VII) ($KMnO_4$).

• Condition: Cold.

• Product: Diol.

• Observation: Purple color of $KMnO_4$ is decolorized, and a brown precipitate ($MnO_2$) forms.

8. Oxidative Cleavage (Strong Oxidation)

• Reagent: Concentrated acidified Potassium Manganate(VII) ($KMnO_4$).

• Condition: Hot (heated under reflux).

• Action: The $C=C$ bond is broken completely.

• Products:

  • Unsubstituted alkene end ($=CH_2$) becomes $CO_2 + H_2O$.

  • Monosubstituted alkene end ($=CHR$) becomes a Carboxylic Acid.

  • Disubstituted alkene end ($=CR_2$) becomes a Ketone.

• Observation: Purple solution is decolorized. This test helps identify the structure and position of the double bond.

9. Polymerization

• Process: Small molecules (monomers) join to form large molecules (polymers).

• Alkenes act as monomers in addition polymerization. The $\pi$ bonds break, allowing monomers to link repeatedly.

• Examples: Polyethylene, polypropylene, polystyrene, poly(vinyl chloride).

Identification Tests: Alkanes vs. Alkenes