Alcohols, Thiols, Ethers, Aldehydes, and Ketones: Structure, Nomenclature, Properties, and Reactions

Alcohols, Thiols, Ethers, Aldehydes, and Ketones

Functional Groups

Organic molecules are classified by their functional groups, which determine their chemical properties and reactivity.

• Alcohol: Contains a hydroxyl group (–OH) attached to a saturated carbon atom.

• Phenol: Contains a hydroxyl group (–OH) bonded to a benzene ring.

• Thiol: Contains a sulfhydryl group (–SH).

• Ether: Contains an oxygen atom bonded to two carbon groups (C–O–C).

Example: Methanol (CH3OH) is a simple alcohol; phenol (C6H5OH) is a simple phenol; ethanethiol (CH3CH2SH) is a thiol; diethyl ether (CH3CH2OCH2CH3) is an ether.

Naming Alcohols

Alcohols are named using both IUPAC and common naming systems.

• In the IUPAC system, replace the e at the end of the parent alkane with ol (e.g., methane → methanol).

• In the common system, use the name of the alkyl group followed by "alcohol" (e.g., methyl alcohol).

Formula	IUPAC Name	Common Name
CH3OH	Methanol	Methyl alcohol
CH3CH2OH	Ethanol	Ethyl alcohol

Example: CH3CH2CH2OH is called 1-propanol (IUPAC) or propyl alcohol (common).

Naming Thiols

Thiols are sulfur-containing organic compounds with a –SH group.

• In the IUPAC system, add "thiol" to the alkane name of the longest carbon chain and number from the end nearer to the –SH group.

• Common names are less frequently used.

Example: CH3CH2SH is called ethanethiol.

Thiols often have strong odors and are found in foods like cheese, onions, and garlic. They are also used to detect gas leaks.

Ethers

Ethers contain an oxygen atom between two carbon groups (alkyl or aromatic).

• Ethers have a bent structure, similar to water and alcohols.

• Common names list the alkyl groups attached to the oxygen in alphabetical order, followed by "ether" (e.g., methyl propyl ether).

Example: CH3OCH2CH2CH3 is methyl propyl ether.

Classification of Alcohols

Alcohols are classified by the number of alkyl groups attached to the carbon bonded to the hydroxyl group.

• Primary (1°) alcohol: The carbon with –OH is attached to one alkyl group.

• Secondary (2°) alcohol: The carbon with –OH is attached to two alkyl groups.

• Tertiary (3°) alcohol: The carbon with –OH is attached to three alkyl groups.

Example: Ethanol is a primary alcohol; 2-propanol is a secondary alcohol; 2-methyl-2-propanol is a tertiary alcohol.

Solubility of Alcohols, Ethers, and Phenols in Water

The solubility of these compounds in water depends on their ability to form hydrogen bonds and the length of their carbon chain.

• Alcohols with 1–3 carbons are soluble in water; solubility decreases as the carbon chain length increases.

• Ethers are generally less soluble than alcohols but can dissolve in water if the carbon chains are short.

• Phenols are slightly soluble in water and can form hydrogen bonds, producing phenoxide ions.

Compound	Condensed Structural Formula	Number of Carbon Atoms	Solubility in Water
Methanol	CH3OH	1	Soluble
Ethanol	CH3CH2OH	2	Soluble
1-Propanol	CH3CH2CH2OH	3	Soluble
1-Butanol	CH3CH2CH2CH2OH	4	Slightly soluble
1-Pentanol	CH3CH2CH2CH2CH2OH	5	Insoluble

Aldehydes and Ketones: The Carbonyl Group

Aldehydes and ketones contain a carbonyl group (C=O), which is a carbon atom double-bonded to an oxygen atom.

• The carbonyl group is polar due to the electronegativity of oxygen.

• In aldehydes, the carbonyl carbon is attached to at least one hydrogen atom.

• In ketones, the carbonyl carbon is attached to two alkyl groups.

Example: Formaldehyde (CH2O) is an aldehyde; acetone (CH3COCH3) is a ketone.

Naming Aldehydes and Ketones

Aldehydes and ketones are named using IUPAC and common systems.

• Aldehydes: Replace the e in the alkane name with al (e.g., methane → methanal). Common names for the first four aldehydes are formaldehyde, acetaldehyde, propionaldehyde, and butyraldehyde.

• Ketones: Replace the e in the alkane name with one (e.g., propane → propanone). Common names list the alkyl groups alphabetically, followed by "ketone" (e.g., methyl ethyl ketone).

Solubility of Aldehydes and Ketones in Water

Aldehydes and ketones can form hydrogen bonds with water due to the polar carbonyl group.

• They are very soluble in water when they have four or fewer carbons.

• Longer hydrocarbon chains decrease solubility due to increased nonpolar character.

Dehydration of Alcohols

Alcohols can undergo dehydration when heated with an acid catalyst, resulting in the formation of an alkene and water.

• The reaction involves the loss of –H and –OH from adjacent carbon atoms.

Equation:

$\text{Alcohol} \xrightarrow{\text{H}^+, \text{heat}} \text{Alkene} + \text{H}_2\text{O}$

Oxidation and Reduction in Organic Chemistry

Oxidation and reduction reactions change the number of carbon–oxygen bonds in organic molecules.

• Oxidation: Increases the number of C–O bonds (addition of oxygen or loss of hydrogen).

• Reduction: Decreases the number of C–O bonds (addition of hydrogen or loss of oxygen).

Oxidation of Alcohols

• Primary (1°) alcohols are oxidized to aldehydes, which can further oxidize to carboxylic acids.

• Secondary (2°) alcohols are oxidized to ketones.

• Tertiary (3°) alcohols do not readily oxidize due to the absence of a hydrogen atom on the carbon bonded to –OH.

Equations:

Primary alcohol to aldehyde: $\text{RCH}_2\text{OH} \xrightarrow{[O]} \text{RCHO}$

Aldehyde to carboxylic acid: $\text{RCHO} \xrightarrow{[O]} \text{RCOOH}$

Secondary alcohol to ketone: $\text{R}_2\text{CHOH} \xrightarrow{[O]} \text{R}_2\text{CO}$

Oxidation of Thiols

Thiols are oxidized to form disulfides, which are important in protein structure (e.g., cysteine cross-links).

Equation:

$2\text{RSH} \xrightarrow{[O]} \text{RSSR} + 2\text{H}^+$

Tollens' Test

Tollens' reagent (Ag+) is used to distinguish aldehydes from ketones.

• Aldehydes are oxidized by Tollens' reagent, producing a silver mirror.

• Ketones do not react.

Equation:

$\text{RCHO} + 2Ag^+ + 2OH^- \rightarrow \text{RCOO}^- + 2Ag(s) + 2H_2O$

Benedict's Test

Benedict's reagent (Cu2+) tests for aldehydes with adjacent hydroxyl groups (e.g., reducing sugars).

• Positive test: Formation of brick-red Cu2O precipitate.

• Negative test: No reaction with simple aldehydes or ketones.

Equation:

$\text{RCHOHCHO} + 2Cu^{2+} + 2H_2O \rightarrow \text{RCOOH} + Cu_2O(s) + 4H^+$

Reduction of Aldehydes and Ketones

Aldehydes and ketones can be reduced to alcohols using sodium borohydride (NaBH4) or hydrogen (H2) with a metal catalyst.

• Aldehydes reduce to primary alcohols.

• Ketones reduce to secondary alcohols.

Equations:

Aldehyde to primary alcohol: $\text{RCHO} + 2[H] \rightarrow \text{RCH}_2\text{OH}$

Ketone to secondary alcohol: $\text{R}_2\text{CO} + 2[H] \rightarrow \text{R}_2\text{CHOH}$

Concept Map: Alcohols, Thiols, Ethers, Aldehydes, and Ketones

This concept map summarizes the relationships between the functional groups and their chemical transformations:

• Alcohols can undergo dehydration (to form alkenes) and oxidation (to form aldehydes or ketones).

• Aldehydes can be further oxidized to carboxylic acids or reduced to primary alcohols.

• Ketones can be reduced to secondary alcohols.

• Thiols can be oxidized to disulfides.

Additional info: These topics are foundational for understanding organic chemistry and biochemistry, including metabolic pathways and laboratory identification of organic compounds.