Alcohols: Properties, Dehydration, Oxidation & Reduction

Exam Details and Alcohols

Exam Information

• The first exam is scheduled for next Friday.

• It will be an online, proctored exam.

• Students must be in a calm environment and take the exam by themselves.

• The exam will be available for a 1-hour window, and students can start it at any time within the designated date.

• Submission is allowed only once.

• The exam will include multiple-choice questions and higher-tier questions requiring written answers (e.g., naming compounds, drawing structures of products).

Physical Properties of Alcohols

Boiling Point

• Alcohols have a significantly higher boiling point compared to alkanes.

• This is attributed to the formation of hydrogen bonds between alcohol molecules.

  • A hydrogen bond forms when a hydrogen atom (attached to a highly electronegative atom like oxygen in alcohols) is attracted to another electronegative atom in an adjacent molecule.

• Ethers, in contrast, have boiling points much closer to alkanes, indicating a smaller gap in physical properties compared to alcohols.

Solubility in Water

• Alcohols are often miscible (soluble) with water due to their ability to form hydrogen bonds with water molecules.

• Effect of Carbon Chain Length:

  • As the carbon chain length (the nonpolar 'tail') of an alcohol increases, the molecule becomes more nonpolar overall.

  • Only the hydroxyl ($-\text{OH}$) group can form hydrogen bonds with water.

  • A long nonpolar carbon chain hinders the alcohol's ability to be dissolved by water.

  • Alcohols with more than five carbon atoms in their structure are generally insoluble in water (e.g., hexanol).

• Effect of Multiple Hydroxyl Groups:

  • If a molecule has multiple hydroxyl groups (e.g., a hexane derivative with -OH on each carbon), its solubility in water significantly increases.

  • Each hydroxyl group can form hydrogen bonds with water, making the molecule more polar and water-soluble despite a long carbon chain.

Reactions of Alcohols: Dehydration

Overview

• Dehydration is a reaction where a molecule of water ($H_2O$) is removed from an alcohol.

• The product of alcohol dehydration is an alkene (a compound with a carbon-carbon double bond).

• This reaction is the reverse of the hydration of alkenes, which produces alcohols.

Conditions and Mechanism

• Reagents: High temperature and a strong absorbing agent such as concentrated sulfuric acid ($H<em>2SO</em>4$).

• Water Removal: Water is removed from two adjacent carbon atoms:

  • One carbon atom must bear the hydroxyl ($-\text{OH}$) group.

  • The adjacent carbon atom must bear a hydrogen atom ($-\text{H}$).

• If no hydrogen is present on an adjacent carbon, the dehydration reaction cannot proceed (e.g., a tertiary alcohol where all adjacent carbons have no hydrogens).

Regioselectivity: Zaitsev's Rule (Rabbit Rule)

• For secondary and tertiary alcohols, dehydration can sometimes lead to the formation of two different alkene products.

• Zaitsev's Rule states that when a choice exists, the major product will be the alkene formed by removing the hydrogen from the adjacent carbon atom that has fewer hydrogen atoms.

  • This typically results in the formation of the more substituted (more stable) alkene as the major product.

• Example for a Secondary Alcohol: If a carbon adjacent to the hydroxyl-bearing carbon has three hydrogens ($CH<em>3$) and another adjacent carbon has two hydrogens ($CH</em>2$), the hydrogen will preferentially be removed from the $CH_2$ group to form the major product.

• The formation of carbocation intermediates is a underlying factor, with more stable carbocations (tertiary > secondary > primary) leading to preferred products, though not detailed for this course.

Dehydration Practice Problems

1. Primary Alcohol Example: A primary alcohol with adjacent carbons having hydrogens will form a single alkene product by removing $-OH$ and $-H$ from adjacent carbons to form a double bond.

2. Secondary Alcohol Example (Zaitsev's Rule): For an alcohol like 3-methylbutan-2-ol:

   • The carbon with the -OH group is a secondary carbon.

   • Adjacent carbons have different numbers of hydrogens (e.g., one has $3$ hydrogens, another has $1$ hydrogen).

   • The hydrogen will be removed from the carbon with fewer hydrogens (e.g., the carbon with $1$ hydrogen) to form the major product.

   • The name 3-methylbutan-2-ol indicates a four-carbon chain with a methyl group on carbon $3$ and an alcohol group on carbon $2$.

3. Equivalent Hydrogens Example: For a symmetrical alcohol where all adjacent carbons have an equal number of hydrogens (e.g., a tertiary alcohol with three equivalent methyl groups adjacent to the carbon bearing the hydroxyl group):

   • Removing a hydrogen from any of these equivalent carbons will yield the same alkene product.

   • The specific example discussed, 2-methylpropan-2-ol, has a central carbon with an -OH group and three adjacent $CH<em>3$ groups. Removal of a hydrogen from any of these $CH</em>3$ groups results in the same product, 2-methylpropene.

Oxidation and Reduction Reactions in Organic Chemistry

Definitions

• Oxidation (General Chemistry): Loss of electrons or an increase in oxidation number.

• Oxidation (Organic Chemistry): An increase in the number of carbon-oxygen bonds ($C-O$) and/or a decrease in the number of carbon-hydrogen bonds ($C-H$).

• Reduction (Organic Chemistry): A decrease in the number of carbon-oxygen bonds ($C-O$) and/or an increase in the number of carbon-hydrogen bonds ($C-H$).

Oxidation State Progression of Carbon

• From lowest to highest oxidation state (and increasing C-O bonds, decreasing C-H bonds):

  • Methane ($CH_4$): Four $C-H$ bonds, zero $C-O$ bonds.

  • Methanol (Alcohol, $-CH_2OH$): Three $C-H$ bonds, one $C-O$ bond.

  • Aldehyde ($-CHO$) or Ketone ($C=O$): Two $C-H$ bonds, two $C-O$ bonds (counting the double bond as two C-O bonds).

  • Carboxylic Acid ($-COOH$): One $C-H$ bond, three $C-O$ bonds.

  • Carbon Dioxide ($CO_2$): Zero $C-H$ bonds, four $C-O$ bonds.

• Any transformation moving from left to right in this series is an oxidation process.

• Any transformation moving from right to left in this series is a reduction process.

Practice Problems: Classifying Reactions

1. Alcohol to Carboxylic Acid Derivative (Example):

   • Starting carbon (in alcohol): $2$ $C-H$ bonds, $1$ $C-O$ bond.

   • Ending carbon (e.g., in a carboxylic acid or its derivative): $0$ $C-H$ bonds, $3$ $C-O$ bonds.

   • Result: Decrease in $C-H$ bonds, increase in $C-O$ bonds. This is an oxidation reaction.

2. Aldehyde/Ketone to Alcohol (Example discussed by students):

   • Starting carbon: e.g., $1$ $C-H$ bond, $2$ $C-O$ bonds (for aldehyde).

   • Ending carbon: e.g., $2$ $C-H$ bonds, $1$ $C-O$ bond (for primary alcohol).

   • Result: Increase in $C-H$ bonds, decrease in $C-O$ bonds. This is a reduction reaction.

3. General Reduction Example: If a transformation leads to an increase in $C-H$ bonds and a decrease in $C-O$ bonds on the relevant carbon atom, it is a reduction.

4. Reaction That is Neither Oxidation Nor Reduction:

   • Consider a reaction where the number of $C-H$ bonds changes, and the number of $C-O$ bonds changes in a way that doesn't fit the clear increase/decrease pattern for oxidation or reduction based on the definitions (e.g., both increase or both decrease in a complex manner).

   • For example, if an alkane is converted to an alkene without involving oxygen, or an addition reaction that doesn't involve changes to C-O bonds, it might not be classified as an oxidation or reduction based on these rules.

   • The key is to evaluate the change in $C-H$ and $C-O$ bonds on the carbon atom(s) undergoing transformation.

   • If the definitions for oxidation or reduction are not met, the reaction is neither; it might be another type of reaction, such as an addition reaction.

Next lecture will begin with exploring specific products formed depending on the class of alcohol involved in these reactions.