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

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CH12: 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 molecular 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): tetrahedral geometry; Ethene (C2H4): planar geometry due to double bond.

IUPAC Nomenclature of Alkanes, Alkenes, and Alkynes

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

  • Alkanes: Saturated hydrocarbons with single bonds. General formula:

  • Alkenes: Unsaturated hydrocarbons with at least one double bond. General formula:

  • Alkynes: Unsaturated hydrocarbons with at least one triple bond. General formula:

  • 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; assemble the name alphabetically.

  • Example: 2-methylpentane, 3-hexene, 2-butyne.

Drawing Organic Structures

  • Expanded Structure: Shows all atoms and bonds explicitly.

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

  • Line-Angle Structure: Each vertex and line end represents a carbon atom; 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. Named with the prefix "cyclo-" (e.g., cyclopentane).

  • Cycloalkenes: Rings containing double bonds (e.g., cyclohexene).

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

Aromatic Compounds (Benzene and Derivatives)

  • Benzene: Aromatic hydrocarbon with a six-membered ring and alternating double bonds (delocalized electrons).

  • Substituted Aromatics: Compounds where one or more hydrogens of benzene are replaced by other groups (e.g., toluene, phenol, aniline).

  • Naming: Use parent names (e.g., methylbenzene for toluene) and indicate substituent positions (ortho-, meta-, para- or numbers).

Addition Reactions of Alkenes

  • Hydrogenation: Addition of H2 across a double bond to form an alkane. Reagent: H2; Catalyst: Pt, Pd, or Ni.

  • Hydration: Addition of H2O across a double bond to form an alcohol. Reagent: H2O; Catalyst: H+ (acid).

  • Polymerization: Linking of many alkene molecules to form polymers (e.g., polyethylene).

  • General Equation for Hydrogenation:

CH13: Alcohols, Phenols, Thiols, and Ethers

Definitions and Structures

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

  • Phenols: Compounds with an -OH group attached directly to a benzene ring.

  • Thiols: Compounds with a sulfhydryl (-SH) group.

  • Ethers: Compounds with an oxygen atom bonded to two alkyl or aryl groups (R-O-R').

Physical Properties

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

  • Solubility: Alcohols (especially small ones) are soluble in water; ethers are moderately soluble; thiols are less soluble.

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

  • Comparison Table:

Compound Type

Boiling Point

Water Solubility

Polarity

Alcohol

High

High (small alcohols)

Polar

Phenol

High

Moderate

Polar

Ether

Moderate

Moderate

Slightly polar

Thiols

Low

Low

Weakly polar

IUPAC Nomenclature

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

  • Phenols: Use "phenol" as the parent name; substituents are named as prefixes.

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

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

Reactions of Alcohols

  • Dehydration: Removal of water to form an alkene. Reagent: H+ (acid), heat.

  • Oxidation: Primary alcohols to aldehydes (then carboxylic acids); secondary alcohols to ketones; tertiary alcohols do not oxidize easily.

  • Combustion: Complete oxidation to CO2 and H2O.

  • Major/Minor Products: In dehydration, the more substituted alkene is the major product (Zaitsev's rule).

  • General Equation for Oxidation: (primary alcohol to aldehyde)

Alcohol Classification

  • 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 with Hydrocarbons

  • Alcohols have higher boiling points and greater water solubility than hydrocarbons of similar molecular weight due to hydrogen bonding.

CH14: 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 (C=O) within the carbon chain (not at the end).

  • General Structures:

    • Aldehyde: R-CHO

    • Ketone: R-CO-R'

Nomenclature

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

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

Identification of Aldehydes and Ketones

  • Tollens’ Test: Aldehydes reduce Tollens’ reagent (Ag(NH3)2+) to metallic silver; ketones do not react.

  • Benedict’s Test: Aldehydes reduce Benedict’s reagent (Cu2+) to a red precipitate; ketones do not react.

Reduction and Oxidation Reactions

  • Reduction of Aldehydes: Aldehydes are reduced to primary alcohols using reducing agents such as NaBH4 or H2/catalyst.

  • Oxidation of Aldehydes: Aldehydes are oxidized to carboxylic acids using oxidizing agents (e.g., KMnO4, K2Cr2O7).

  • General Equations:

    • Reduction:

    • Oxidation:

Reactions with Alcohols

  • Hemiacetal Formation: Aldehyde or ketone reacts with one equivalent of alcohol.

  • Acetal Formation: Hemiacetal reacts with a second equivalent of alcohol (acid catalyst required).

  • General Equation:

Physical Properties

  • Boiling Points: Higher than hydrocarbons and ethers, lower than alcohols (due to dipole-dipole interactions, but no hydrogen bonding).

  • Solubility: Small aldehydes and ketones are soluble in water; solubility decreases with increasing chain length.

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