Alcohol Chemistry

Naming Alcohols

• General formula: $C<em>nH</em>{2n+1}OH$
• Ending: -ol
• Position number: Added between the name stem and the -ol if necessary (e.g., butan-2-ol).
• Prefix hydroxy-: Used when the -OH group is in addition to other functional groups that need a suffix ending (e.g., 2-hydroxypropanoic acid).
• Multiple -OH groups: Use di, tri, etc. Add the ‘e’ on to the stem name (e.g., ethane-1,2-diol, propane-1,2,3-triol).

Types of Alcohols

• Primary alcohols: One carbon attached to the carbon adjoining the oxygen (e.g., propan-1-ol).
• Secondary alcohols: Two carbons attached to the carbon adjoining the oxygen (e.g., propan-2-ol).
• Tertiary alcohols: Three carbons attached to the carbon adjoining the oxygen (e.g., methylpropan-2-ol).

Bond Angles in Alcohols

• H-C-H bonds and C-C-O: 109.5° (tetrahedral shape)
  • Reason: 4 bonding pairs of electrons repelling to a position of minimum repulsion.
• H-O-C bond: 104.5° (bent line shape)
  • Reason: 2 bonding pairs of electrons and 2 lone pairs repelling to a position of minimum repulsion.
  • Lone pairs repel more than bonding pairs, so the bond angle is reduced.

Physical Properties of Alcohols

• Volatility: Relatively low due to hydrogen bonding between alcohol molecules.
• Boiling points: High due to hydrogen bonding.
• Solubility: Smaller alcohols can dissolve in water because they can form hydrogen bonds to water molecules.

Oxidation Reactions of Alcohols

• Oxidizing agent: Potassium dichromate ($K<em>2Cr</em>2O_7$).
• Reaction depends on the type of alcohol (primary, secondary, or tertiary) and the conditions.

Partial Oxidation of Primary Alcohols

• Reaction: primary alcohol -> aldehyde
• Reagent: potassium dichromate(VI) solution and dilute sulfuric acid
• Conditions: limited amount of dichromate, warm gently, and distill out the aldehyde as it forms.
• Aldehyde name ends in -al. The C=O bond is always on the first carbon of the chain, so no number is needed in its name (e.g., ethanal).
• Observation: orange dichromate ion ($Cr<em>2O</em>7^{2-}$) reduces to the green $Cr^{3+}$ ion.
• Oxidation equations use [O] to represent O from the oxidizing agent.
• Condensed formulae of aldehydes are written as CHO, not COH (e.g., $CH<em>3CH</em>2CHO$).
  • Example: propan-1-ol -> propanal
  • $CH<em>3CH</em>2CH<em>2OH + [O] \rightarrow CH</em>3CH<em>2CHO + H</em>2O$

Distillation

• Technique: used to separate an organic product from its reacting mixture.
• Maximizing yield: collect the distillate at the approximate boiling point of the desired aldehyde and not higher.
• Apparatus:
  • Liebig condenser
  • Thermometer (bulb at the T junction)
  • Round-bottomed flask
  • Electric heater (safer for flammable organic chemicals)
  • Collection flask (cooled in ice to improve yield).
• Water flow: into the bottom of the condenser to go against gravity for efficient cooling and prevents backflow of water.
• Important: accurately draw and label the apparatus; no lines between flask, adapter, and condenser, or across the thermometer.

Full Oxidation of Primary Alcohols

• Reaction: primary alcohol -> carboxylic acid
• Reagent: potassium dichromate(VI) solution and dilute sulfuric acid
• Conditions: excess of dichromate, and heat under reflux; distil off product after the reaction has finished.
• Observation: orange dichromate ion ($Cr<em>2O</em>7^{2-}$) reduces to the green $Cr^{3+}$ ion.
  • Example: propan-1-ol -> propanoic acid
  • $CH<em>3CH</em>2CH<em>2OH + 2[O] \rightarrow CH</em>3CH<em>2COOH + H</em>2O$

Reflux

• Use: heating organic reaction mixtures for long periods.
• Condenser: prevents organic vapors from escaping by condensing them back to liquids.
• Safety: never seal the end of the condenser to prevent pressure buildup and potential explosion.
• Anti-bumping granules: added to prevent vigorous, uneven boiling by making small bubbles form instead of large bubbles.
• Apparatus:
  • Round bottomed flask
  • Condenser (with outer tube for water, open at top and bottom)
  • Water in and out openings

Oxidation of Secondary Alcohols

• Reaction: secondary alcohol -> ketone
• Reagent: potassium dichromate(VI) solution and dilute sulfuric acid
• Conditions: heat under reflux
• Ketones end in -one. If there are 5 or more carbons, a number is needed to indicate the position of the double bond (e.g., pentan-2-one).
  • Example: propan-2-ol -> propanone
  • $CH<em>3CHOHCH</em>3 + [O] \rightarrow CH<em>3COCH</em>3 + H_2O$
• No further oxidation of the ketone occurs under these conditions.
• Observation: The orange dichromate ion ($Cr<em>2O</em>7^{2-}$) reduces to the green $Cr^{3+}$ ion.

Tertiary Alcohols

• Cannot be oxidized by potassium dichromate because there is no hydrogen atom bonded to the carbon with the -OH group.

Distinguishing Between Aldehydes and Ketones

• Aldehydes can be further oxidized to carboxylic acids, whereas ketones cannot.
• Tollens’ Reagent
  • Reagent: formed by mixing aqueous ammonia and silver nitrate. The active substance is the complex ion of $[Ag(NH<em>3)</em>2]^+$.
  • Conditions: heat gently
  • Reaction: aldehydes only are oxidized by Tollens’ reagent into a carboxylic acid. The silver(I) ions are reduced to silver atoms
  • Observation: with aldehydes, a silver mirror forms coating the inside of the test tube. Ketones result in no visible change.
    • $CH<em>3CHO + 2Ag^+ + H</em>2O \rightarrow CH_3COOH + 2Ag + 2H^+$
• Fehling’s Solution
  • Reagent: containing blue $Cu^{2+}$ ions.
  • Conditions: heat gently
  • Reaction: aldehydes only are oxidized by Fehling’s solution into a carboxylic acid. The copper (II) ions are reduced to copper(I) oxide.
  • Observation: Aldehydes: Blue $Cu^{2+}$ ions in solution change to a red precipitate of $Cu_2O$. Ketones do not react.
    • $CH<em>3CHO + 2Cu^{2+} + 2H</em>2O \rightarrow CH<em>3COOH + Cu</em>2O + 4H^+$

Carboxylic Acid Test

• Addition of sodium carbonate: If a carboxylic acid is present, it will fizz and produce carbon dioxide.

Reaction of Alcohols with Dehydrating Agents

Dehydration

• Reaction: removal of a water molecule from a molecule
• Reaction: Alcohol -> Alkene
• Reagents: Concentrated sulfuric or phosphoric acids
• Conditions: warm (under reflux)
• Role of reagent: dehydrating agent/catalyst
• Type of reaction: acid catalyzed elimination
  • Example: propan-1-ol -> propene
  • $CH<em>3CH</em>2CH<em>2OH \rightarrow CH</em>2=CHCH<em>3 + H</em>2O$
• Some 2° and 3° alcohols can give more than one product, when the double bond forms between different carbon atoms.
• Example: butan-2-ol can form both but-1-ene and but-2-ene, although more but-2-ene would be formed.
• But-2-ene could also exist as E and Z isomers
• Producing alkenes from alcohols provides a possible route to polymers without using monomers derived from oil.
• Acid Catalyzed Elimination Mechanism
  • The $H^+$ comes from the conc $H<em>2SO</em>4$ or conc $H<em>3PO</em>4$

Forming Ethanol - Two Methods

Fermentation

• Reaction: glucose -> ethanol + carbon dioxide
  • $C<em>6H</em>{12}O<em>6 \rightarrow 2CH</em>3CH<em>2OH + 2CO</em>2$
• Conditions:
  • Yeast
  • No air
  • Temperatures 30 – 40°C
  • Optimum temperature: around 38°C (lower temperatures slow the reaction; higher temperatures kill the yeast and denature the enzymes).
• Absence of air: prevents oxidation of ethanol to ethanoic acid (vinegar).
• Advantages:
  • Sugar is a renewable resource
  • Low-level technology/cheap equipment
• Disadvantages:
  • Batch process which is slow and gives high production costs
  • Ethanol made is not pure and needs purifying by fractional distillation
  • Depletes land used for growing food crops

Hydration of Ethene

• Reaction: ethene(g) + water(g) -> ethanol(l)
  • $CH<em>2=CH</em>2(g) + H<em>2O(g) \rightarrow CH</em>3CH_2OH(l)$
• Conditions:
  • High temperature: 300 °C
  • High pressure: 70 atm
  • Strong acidic catalyst: conc $H<em>3PO</em>4$
• Reagent: Ethene - from cracking of fractions from distilled crude oil
• Advantages:
  • Faster reaction
  • Purer product
  • Continuous process (cheaper manpower)
• Disadvantages:
  • High-technology equipment needed (expensive initial costs)
  • Ethene is a non-renewable resource (will become more expensive when raw materials run out)
  • High energy costs for pumping to produce high pressures
• Type of reaction:
  • Fermentation
  • Hydration/addition
    *Definition: Hydration is the addition of water to a molecule
• Acid Catalyzed Addition Mechanism for Hydration of Ethene
  • The $H^+$ comes from the conc $H<em>3PO</em>4$

Ethanol as Biofuel

• Biofuel: a fuel produced from plants
• Argument for ethanol as carbon-neutral:
  • $CO_2$ given off when the biofuel is burned would have been extracted from the air by photosynthesis when the plant grew.
  • No net $CO_2$ emission into the atmosphere.
• Considerations:
  • Energy needed to irrigate plants, fractionally distill ethanol, and process the fuel.
  • If energy for these processes comes from fossil fuels, then the ethanol produced is not carbon neutral.

Equations

• Removal of $CO_2$ by photosynthesis:
  • $6CO<em>2 + 6H</em>2O \rightarrow C<em>6H</em>{12}O<em>6 + 6O</em>2$
• Production of $CO_2$ by fermentation and combustion:
  • $C<em>6H</em>{12}O<em>6 \rightarrow 2CH</em>3CH<em>2OH + 2CO</em>2$
  • $2CH<em>3CH</em>2OH + 6O<em>2 \rightarrow 4CO</em>2 + 6H_2O$
• Overall for every 6 molecules of $CO<em>2$ absorbed, 6 molecules of $CO</em>2$ are emitted. There is no net contribution of $CO_2$ to the atmosphere.
• Carbon neutral: “an activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere”