CHM132 Exam 1 Study Guide: Hydrocarbons, Alcohols, Aldehydes & Ketones

Hydrocarbons

Organic Compounds: Structure and Bonding

Organic compounds are molecules primarily composed of carbon and hydrogen, often containing other elements such as oxygen, nitrogen, sulfur, and halogens. The unique bonding properties of carbon allow for a vast diversity of structures.

• Carbon Bonding: Carbon forms four covalent bonds, resulting in tetrahedral geometry (bond angles of 109.5°) in alkanes.

• Hybridization: Carbon atoms in alkanes are sp3 hybridized, in alkenes sp2, and in alkynes sp.

• Structural Representation: Organic molecules can be depicted as expanded, condensed, or line-angle (skeletal) structures.

• Example: Methane (CH4) is the simplest alkane, with a tetrahedral geometry.

IUPAC Nomenclature of Alkanes, Alkenes, and Alkynes

The International Union of Pure and Applied Chemistry (IUPAC) system provides rules for naming organic compounds systematically.

• Alkanes: Saturated hydrocarbons with only single bonds. General formula: $C_nH_{2n+2}$

• Alkenes: Unsaturated hydrocarbons with at least one double bond. General formula: $C_nH_{2n}$

• Alkynes: Unsaturated hydrocarbons with at least one triple bond. General formula: $C_nH_{2n-2}$

• Substituents: Groups attached to the main chain (e.g., methyl, ethyl).

• Naming Steps:

  1. Identify the longest carbon chain (parent chain).

  2. Number the chain to give the lowest possible numbers to double/triple bonds and substituents.

  3. Name and number substituents; list alphabetically.

  4. Combine substituent names, numbers, and parent name.

• Example: 2-methylpropane (isobutane)

Drawing Structures

• Expanded Structure: Shows all atoms and bonds explicitly.

• Condensed Structure: Groups atoms together (e.g., CH3CH2CH3).

• Line-Angle Structure: Each vertex or line end represents a carbon; hydrogens on carbons are implied.

Properties of Hydrocarbons

• Boiling Points: Increase with molecular weight and surface area; decrease with branching.

• Physical States: Small alkanes are gases, medium are liquids, large are solids at room temperature.

• Solubility: Hydrocarbons are nonpolar and insoluble in water; soluble in nonpolar solvents.

• Polarity: Hydrocarbons are generally nonpolar due to similar electronegativities of C and H.

Cis-Trans Isomers and Structural Isomers

• Structural Isomers: Compounds with the same molecular formula but different connectivity.

• Cis-Trans (Geometric) Isomers: Occur in alkenes when each carbon of the double bond has two different groups. Cis = same side; Trans = opposite sides.

• Example: 2-butene exists as cis-2-butene and trans-2-butene.

Naming Cyclic Alkanes and Alkenes

• Cycloalkanes: Saturated hydrocarbons with carbon atoms arranged in a ring. Prefix "cyclo-" is used.

• Cycloalkenes: Rings containing double bonds; number the ring to give the double bond the lowest possible numbers.

• Drawing: Use polygons for rings in line-angle structures.

• Example: Cyclohexane (C6H12), cyclopentene (C5H8).

Aromatic Compounds (Benzene and Derivatives)

• Benzene: Aromatic hydrocarbon with formula C6H6; six-membered ring with alternating double bonds (delocalized electrons).

• Substituted Aromatics: Benzene rings with one or more substituents (e.g., methylbenzene = toluene).

• Common Names: Aniline (aminobenzene), phenol (hydroxybenzene), toluene (methylbenzene).

Naming Substituted Aromatic Compounds

• Ortho- (o-), Meta- (m-), Para- (p-): Indicate relative positions of substituents (1,2-; 1,3-; 1,4-).

• Example: p-dichlorobenzene (1,4-dichlorobenzene)

Addition Reactions of Alkenes

Alkenes undergo addition reactions where atoms add across the double bond.

• Hydrogenation: Addition of H2 across the double bond; requires a metal catalyst (e.g., Pt, Pd, Ni).

• Hydration: Addition of H2O (as H and OH) across the double bond; requires acid catalyst (e.g., H2SO4).

• Polymerization: Many alkene molecules join to form a polymer (e.g., ethene to polyethylene).

• General Equation for Hydrogenation: $\ce{RCH=CHR' + H2 ->[Pt] RCH2CH2R'}$

Alcohols, Phenols, Thiols, and Ethers

Definitions and Structures

• Alcohols: Organic compounds with a hydroxyl (-OH) group attached to a saturated carbon.

• Phenols: Hydroxyl group attached directly to a benzene ring.

• Thiols: Contain a sulfhydryl (-SH) group.

• Ethers: Oxygen atom bonded to two alkyl or aryl groups (R-O-R').

Physical Properties

• Boiling Points: Alcohols and phenols have higher boiling points than ethers and thiols due to hydrogen bonding.

• Solubility: Alcohols with short chains are soluble in water; solubility decreases with chain length.

• Polarity: Alcohols and phenols are polar; ethers are less polar; thiols are weakly polar.

IUPAC Nomenclature

• Alcohols: Replace "-e" of alkane with "-ol" (e.g., ethanol).

• Phenols: Use "phenol" as parent; name substituents.

• Thiols: Add "thiol" to parent name (e.g., methanethiol).

• Ethers: Name each alkyl group, then add "ether" (common); IUPAC: use "alkoxyalkane" (e.g., methoxyethane).

Reactions of Alcohols

• Dehydration: Alcohol loses H2O to form an alkene; requires acid catalyst and heat. $\ce{RCH2CH2OH ->[H2SO4, \Delta] RCH=CH2 + H2O}$

• Oxidation: Alcohol is oxidized to aldehyde/ketone or carboxylic acid (depends on class).

  • 1° alcohol: $\ce{RCH2OH ->[O] RCHO ->[O] RCOOH}$

  • 2° alcohol: $\ce{R2CHOH ->[O] R2CO}$

  • 3° alcohol: No reaction under mild conditions.

• Combustion: Complete oxidation to CO2 and H2O. $\ce{C2H5OH + 3O2 -> 2CO2 + 3H2O}$

Major/Minor Products

• In dehydration and some oxidation reactions, more than one product may form; the major product is usually the more substituted (Zaitsev's rule).

Classification of Alcohols

• Primary (1°): -OH group attached to a carbon bonded to one other carbon.

• Secondary (2°): -OH group attached to a carbon bonded to two other carbons.

• Tertiary (3°): -OH group attached to a carbon bonded to three other carbons.

Comparison: Boiling Points and Solubility

Compound Type	Boiling Point	Solubility in Water
Alcohols	High (hydrogen bonding)	High (short chains)
Hydrocarbons	Low	Insoluble
Ethers	Intermediate	Moderate
Thiols	Low	Low

Aldehydes and Ketones

Definitions and Structures

• Aldehydes: Organic compounds with a carbonyl group (C=O) at the end of the carbon chain.

• Ketones: Organic compounds with a carbonyl group within the carbon chain (not at the end).

• General Structures:

  • Aldehyde: $\ce{RCHO}$

  • Ketone: $\ce{RCOR'}$

Nomenclature

• Aldehydes: Replace "-e" of alkane with "-al" (e.g., ethanal).

• Ketones: Replace "-e" of alkane with "-one" (e.g., propanone).

Identification: Tollens' and Benedict's Tests

• Tollens' Test: Aldehydes reduce Ag+ to metallic silver; ketones do not react.

• Benedict's Test: Aldehydes (and some alpha-hydroxy ketones) reduce Cu2+ to red Cu2O precipitate.

Reduction and Oxidation Reactions

• Reduction of Aldehydes: Forms primary alcohols; requires reducing agent (e.g., H2/Pt or NaBH4). $\ce{RCHO + H2 ->[Pt] RCH2OH}$

• Oxidation of Aldehydes: Forms carboxylic acids; requires oxidizing agent (e.g., KMnO4, K2Cr2O7). $\ce{RCHO + [O] -> RCOOH}$

• Ketones: Generally do not oxidize further under mild conditions.

Reactions with Alcohols

• Aldehyde/Ketone + Alcohol: Forms hemiacetal (one equivalent alcohol), then acetal (two equivalents alcohol) under acid catalysis.

• General Equations:

  • Hemiacetal formation: $\ce{RCHO + R'OH <=> RCH(OH)OR'}$

  • Acetal formation: $\ce{RCH(OH)OR' + R'OH <=> RCH(OR')2 + H2O}$

Physical Properties

• Boiling Points: Lower than alcohols (no hydrogen bonding), higher than alkanes/ethers of similar mass.

• Solubility: Small aldehydes and ketones are soluble in water due to hydrogen bonding with water; solubility decreases with chain length.