Organic chemistry — alkanes, alkenes, alcohols (KCSE Chemistry Form 4)

TL;DR

Organic chemistry is about carbon compounds, and we'll focus on three main families: alkanes (single bonds), alkenes (double bonds), and alcohols (contain -OH group). Understanding their structures, naming, and reactions is key to mastering this topic. These concepts build on each other, so getting the basics right is super important.

1. The Mental Model

Think of organic chemistry as building with LEGO bricks, where carbon atoms are the main blocks. How these blocks connect (single or double bonds) and what other small pieces (like oxygen and hydrogen) are attached determines the molecule's family and how it behaves.

2. The Core Material

What is Organic Chemistry?

Organic chemistry is the study of compounds containing carbon. Carbon is special because it can form four bonds and link up with itself in long chains, rings, and branched structures, creating a huge variety of molecules.

Hydrocarbons: Alkanes and Alkenes

Hydrocarbons are organic compounds made only of carbon and hydrogen atoms.

Alkanes

Alkanes are saturated hydrocarbons, meaning they only have single covalent bonds between carbon atoms. Their general formula is CnH2n+2, where 'n' is the number of carbon atoms.

• Naming Alkanes:
  • The name ends in "-ane".
  • The prefix tells you the number of carbon atoms:
    • 1 carbon: Meth- (e.g., Methane, CH4)
    • 2 carbons: Eth- (e.g., Ethane, C2H6)
    • 3 carbons: Prop- (e.g., Propane, C3H8)
    • 4 carbons: But- (e.g., Butane, C4H10)
    • 5 carbons: Pent- (e.g., Pentane, C5H12)
    • And so on...
• Properties of Alkanes:
  • Generally unreactive due to strong C-C and C-H single bonds.
  • Undergo combustion (burn in oxygen to produce CO2 and H2O).
  • Undergo substitution reactions with halogens (like chlorine) in the presence of UV light, where a hydrogen atom is replaced by a halogen atom.

Alkenes

Alkenes are unsaturated hydrocarbons, meaning they contain at least one carbon-carbon double covalent bond. Their general formula is CnH2n (for alkenes with one double bond).

• Naming Alkenes:
  • The name ends in "-ene".
  • The prefix is the same as for alkanes, indicating the number of carbon atoms.
  • The position of the double bond needs to be indicated if there are more than 3 carbons (e.g., But-1-ene, But-2-ene).
  • Smallest alkene: Ethene (C2H4) – you can't have a double bond with only one carbon!
• Properties of Alkenes:
  • More reactive than alkanes due to the presence of the double bond.
  • Undergo combustion.
  • Undergo addition reactions, where atoms are added across the double bond, breaking it and forming a single bond. Examples:
    • Hydrogenation: Adding H2 (with Ni catalyst) to form an alkane.
    • Halogenation: Adding halogens (like Br2) to form a haloalkane. This is used to test for unsaturation (bromine water changes from orange-brown to colourless).
    • Hydrohalogenation: Adding HX (like HCl) to form a haloalkane.
    • Hydration: Adding H2O (steam, with H3PO4 catalyst) to form an alcohol.
  • Undergo polymerization, where many small alkene molecules (monomers) join to form a large polymer.

Alcohols

Alcohols are organic compounds containing the hydroxyl functional group (-OH) attached to a carbon atom. Their general formula is CnH2n+1OH.

• Naming Alcohols:
  • The name ends in "-ol".
  • The prefix indicates the number of carbon atoms.
  • The position of the -OH group needs to be indicated if there are more than 2 carbons.
  • Examples:
    • Methanol (CH3OH)
    • Ethanol (C2H5OH)
    • Propan-1-ol (CH3CH2CH2OH)
    • Propan-2-ol (CH3CH(OH)CH3)
• Properties of Alcohols:
  • The -OH group makes them polar, so smaller alcohols are soluble in water.
  • Have higher boiling points than corresponding alkanes due to hydrogen bonding.
  • Undergo combustion.
  • Undergo oxidation:
    • Primary alcohols (like ethanol) can be oxidized to aldehydes, then to carboxylic acids.
    • Secondary alcohols (like propan-2-ol) can be oxidized to ketones.
    • Tertiary alcohols are resistant to oxidation.
  • Undergo dehydration (elimination of water) to form alkenes, usually with concentrated H2SO4 as a catalyst.

Distinguishing Alkanes, Alkenes, and Alcohols

Here's a flowchart to help you distinguish between these three families using simple chemical tests:

graph TD
    A[Unknown Organic Compound] --> B{Add Bromine Water?};
    B -- Decolorizes (Orange-brown to Colorless) --> C[Alkene];
    B -- No Change --> D{Add Acidified Potassium Dichromate(VI)?};
    D -- Changes from Orange to Green --> E[Alcohol];
    D -- No Change --> F[Alkane];

3. Worked Example

Problem: You are given three unlabelled test tubes, each containing a different liquid: hexane, hex-1-ene, and hexan-1-ol. Describe how you would identify each liquid using simple chemical tests.

Solution:

1. Test for Unsaturation (Alkene):

   • Take a small sample from each test tube into separate clean test tubes.
   • Add a few drops of bromine water (orange-brown solution) to each sample.
   • Observation:
     • The liquid that causes the bromine water to decolorize immediately (turn colourless) is hex-1-ene. This is because the double bond in the alkene undergoes an addition reaction with bromine.
     • The other two liquids will show no immediate change; the bromine water layer will remain orange-brown.

2. Test for Alcohol:

   • Take a small sample from the remaining two unlabelled test tubes (hexane and hexan-1-ol) into separate clean test tubes.
   • Add a few drops of acidified potassium dichromate(VI) solution (orange solution) to each, and gently warm the mixture.
   • Observation:
     • The liquid that causes the orange potassium dichromate(VI) solution to turn green is hexan-1-ol. This is because the alcohol is oxidized by the dichromate(VI) ions, which are themselves reduced to green Cr3+ ions.
     • The other liquid will show no change; the solution will remain orange.

3. Identification of Alkane:

   • The remaining liquid, which did not decolorize bromine water and did not change the colour of acidified potassium dichromate(VI), is hexane. Alkanes are generally unreactive to these reagents under normal conditions.

4. Key Takeaways

• Alkanes are saturated hydrocarbons with only C-C single bonds, following CnH2n+2.
• Alkenes are unsaturated hydrocarbons with at least one C=C double bond, following CnH2n.
• Alcohols contain the -OH functional group, making them more polar and reactive than alkanes.
• Alkenes are identified by their ability to decolorize bromine water due to addition reactions.
• Alcohols can be identified by their ability to be oxidized by acidified potassium dichromate(VI) (turning orange to green).
• Alkanes are generally unreactive to these common reagents.

Common Mistakes to Avoid:
- Confusing the general formulas for alkanes (CnH2n+2) and alkenes (CnH2n).
- Forgetting to specify the position of the double bond or -OH group when naming longer chains.
- Assuming all alcohols react the same way to oxidation; primary, secondary, and tertiary alcohols behave differently.
- Not understanding why a reaction occurs (e.g., double bond reactivity in alkenes).

5. Now Try It

Exercise:
Draw the full structural formula for but-1-ene and butan-2-ol. Then, describe the reagents and conditions needed to convert but-1-ene into butan-1-ol, and explain the type of reaction involved.

What success looks like:
You'll have correctly drawn both structures, showing all atoms and bonds. You'll identify that hydration (addition of steam) is needed to convert but-1-ene to butan-1-ol, specifying concentrated phosphoric(V) acid (H3PO4) and high temperature/pressure as conditions. You'll also correctly state that this is an addition reaction.