Organic Chemistry - Chapter 20

Organic Chemistry

Chapter 20 Part 1

Text Sections

• 20.1 Hydrocarbons
• General Chemistry II, 2024 Professor Maggie Ciszkowska

The Structures of Organic Molecules

• Organic molecules are compounds primarily made up of carbon, often combined with hydrogen (H), oxygen (O), nitrogen (N), sulfur (S), and some other elements.
• The carbon atoms within organic molecules can exhibit three types of hybridization:
  • sp³ hybridized carbons: results in tetrahedral geometries.
  • sp² hybridized carbons: results in trigonal planar geometries.
  • sp hybridized carbons: results in linear geometries.

Typical Number of Bonds in Organic Molecules

• For common elements found in organic molecules, the typical number of bonds is as follows:
  • C (carbon): 4 bonds
  • O (oxygen): 2 bonds
  • N (nitrogen): 3 bonds
  • F, Cl, Br, I (halogens): 1 bond
  • H (hydrogen): 1 bond

The Stabilities of Organic Molecules

• Carbon forms very strong bonds with:

  • Hydrogen (H)
  • Oxygen (O)
  • Nitrogen (N)
  • Halogens (F, Cl, Br, I)

• Carbon also forms strong bonds with itself, enabling the formation of stable long-chain or ring structures.

• Bond strength increases in the order of single < double < triple bonds.

• Conversely, bond length decreases in that same order.

• The C-H bond is nearly non-polar due to the similar electronegativities of carbon and hydrogen, which contributes to the stability of compounds rich in C-C and C-H bonds.

• The introduction of functional groups (e.g., C-O-H) to organic molecules increases their reactivity.

Solubility and Acid-Base Properties of Organic Substances

• Organic compounds comprising only C-C or C-H bonds are nonpolar; thus:

  • They are soluble in nonpolar solvents.
  • They have low solubility in water.

• Organic molecules with polar functional groups are water-soluble.

• Surfactants have long nonpolar portions with a small ionic or polar tip.

• The most significant organic acids are carboxylic acids that contain the -COOH functional group.

• Common basic organic molecules are amines, which include functional groups like -NH₂, -NHR, or -NR₂.

Introduction to Hydrocarbons

• Hydrocarbons are defined as compounds containing solely carbon (C) and hydrogen (H).
• Saturated hydrocarbons: Consist of only single bonds (s bonds).
• Unsaturated hydrocarbons: Contain both single (s) and double/triple (p) bonds.
• Four classes of hydrocarbons:
  • Alkanes: Only single bonds.
  • Alkenes: A mix of single and double bonds.
  • Alkynes: Must contain at least one triple bond.
  • Aromatics: Have planar ring structures with delocalized electrons.

Structural Examples of Hydrocarbons

• ### Alkanes: CₙH₂ₙ₊₂
  • Ethane: $ ext{CH}3 ext{CH}3$, sp³ hybridized, 1.54 Å bond length, tetrahedral geometry with 109.5° bond angle.
  • ### Alkenes: CₙH₂ₙ
  • Ethylene: $ ext{C}2 ext{H}4$ or $ ext{CH}2= ext{CH}2$, sp² hybridized, 1.34 Å bond length, trigonal planar geometry with 120° bond angle.
  • ### Alkynes: CₙH₂ₙ₋₂
  • Acetylene: $ ext{C}2 ext{H}2$ or $ ext{HC} ext{≡} ext{CH}$, sp hybridized, 1.21 Å bond length, linear geometry with 180° bond angle.
  • ### Aromatic: Benzene (C₆H₆)
  • Displays delocalized electrons, sp² hybridized carbon atoms.

Properties of Alkanes

• Molecular Formula for Alkanes: CₙH₂ₙ₊₂

• Table of First Several Members of the Straight-Chain Alkane Series:

  • Methane (CH₄): bp -161°C
  • Ethane (C₂H₆): bp -89°C
  • Propane (C₃H₈): bp -44°C
  • Butane (C₄H₁₀): bp -0.5°C
  • Pentane (C₅H₁₂): bp 36°C
  • Hexane (C₆H₁₄): bp 68°C
  • Heptane (C₇H₁₆): bp 98°C
  • Octane (C₈H₁₈): bp 125°C
  • Nonane (C₉H₂₀): bp 151°C
  • Decane (C₁₀H₂₂): bp 174°C

• The naming of alkanes is based on the number of carbon atoms in the chain.

• Because they only exhibit London dispersion forces, boiling points increase smoothly with an increase in molar mass.

• Physical states at normal pressure and 25 °C:

  • Methane to butane are gases.
  • Pentane to decane are liquids.
  • Alkanes with more than 10 carbon atoms are typically solids.

Structural Isomers of Alkanes

• Each carbon in an alkane engages in four single bonds (s bonds).
• Alkanes differ by one CH₂ unit, forming a homologous series.
• Straight Chain Hydrocarbons: Carbons linked in a continuous chain are not linear; each carbon is tetrahedral, resulting in bent chains.
• Branched Chain Hydrocarbons: Possible with four or more carbons, leading to structural isomers which possess varying physical properties.

Examples of Structural Isomers of Butane and Pentane

• Butane

  • Straight chain: $ ext{CH}3 ext{CH}2 ext{CH}2 ext{CH}3$ (bp -0.5°C)
  • Isobutane (2-methylpropane): $ ext{C}4 ext{H}{10}$ (bp -10°C)

• Pentane:

  • Straight chain: $ ext{CH}3 ext{CH}2 ext{CH}2 ext{CH}2 ext{CH}_3$ (bp +36°C)
  • Isopentane (2-methylbutane): (bp +28°C)
  • Neopentane (2,2-dimethylpropane): (bp +9°C)

Naming of Alkanes

• To name alkanes:
  • Identify the longest carbon chain as the base name (e.g., propane for 3 C atoms).
  • Number the carbons from the end nearest to the substituent, e.g., 2-methylpropane.
  • Name all substituents and list in alphabetical order.

Common Substituent Names

• -CH₃: Methyl
• -C₂H₅: Ethyl
• -C₃H₇: Propyl
• -C₄H₉: Butyl

Cycloalkanes

• Cycloalkanes are alkanes that form rings.
• Cyclopropane and cyclobutane are strained due to bond angles deviating from the required tetrahedral angle of 109.5°, making them more reactive compared to unstrained alkanes.
• Cycloalkanes examples include: cyclopropane (C₃H₆), cyclobutane (C₄H₈), and cyclohexane (C₆H₁₂).

Reactions of Alkanes

• Alkanes are generally unreactive at room temperature and do not react with acids, bases, or strong oxidizing agents.
• They can combust in the presence of air:
  • 2 $ ext{C}2 ext{H}6$ + 7 $ ext{O}2$ → 4 $ ext{CO}2$ + 6 $ ext{H}_2 ext{O}$; $ ext{Δ}H = -2855 ext{kJ}$.

Alkenes and Alkynes

Alkenes: CₙH₂ₙ

• Alkenes contain carbon (C) and hydrogen (H), consisting of single and double bonds.
• Base naming is similar to that of alkanes but uses the suffix -ene (e.g., ethylene or ethene $ ext{C}2 ext{H}4$).
• The double bond location is indicated by a number (e.g., 1-butene).

Alkynes: CₙH₂ₙ₋₂

• Alkynes contain at least one triple bond, similarly named using the suffix -yne (e.g., acetylene or ethyne).

Reactions of Alkenes and Alkynes

• Basic reactions for hydrocarbons include:
  • Combustion Reaction: Involves burning in O₂.
  • Addition Reaction: Involves adding elements to the unsaturated hydrocarbon.
  • Polymerization Reaction: Formation of polymers from monomers.

Addition Reactions for Alkenes and Alkynes

• Addition of halogens (e.g. $ ext{Br}_2$):
  • H₂C=CH₂ + Br₂ → H₂C(Br)CH₂(Br)
• Addition of hydrogen (hydrogenation):
  • CH₃CH=CH₂ + H₂ → CH₃CH₂CH₃
• Addition of hydrogen halides:
  • CH₂=CH₂ + HBr → CH₃CH₂Br
  • CH₂=CH₂ + H₂O → CH₃CH₂OH

Polymerization Reaction

• Involves linking unsaturated hydrocarbons together; examples include the formation of plastics.

Aromatic Hydrocarbons

• Aromatic structures relate to benzene, characterized by delocalized p electrons.
• Benzene (C₆H₆) is stable due to these electrons; it does not readily undergo typical reactions of alkenes.

Functional Groups in Organic Compounds

• Reactivity in organic molecules mostly arises from the presence of functional groups, which include elements beyond C or H (e.g., O, N).


• Functional groups are classified in the following table, illustrating the compound type, suffix or prefix, and examples:
  

• Alcohols: Exhibit -OH groups, are typically more soluble in water than alkanes due to O-H bond polarity.
  • Common examples include methanol as a fuel additive.
• Production involves synthesizing from carbon monoxide and hydrogen or via fermentation.

Overview of Ethers and Carbonyl Compounds

• Ethers: Compounds featuring an ether function (R-O-R¹), commonly used as solvents.
• Carbonyl Functionality: Key functional group in molecules such as aldehydes and ketones, with reactions initiated via oxidization of alcohols.

Carboxylic Acids and Esters

• Carboxylic acids feature a -COOH group and typically exhibit weak acidity.
  • Can be prepared via the oxidation of alcohols.
• Esters: Formed from carboxylic acids and alcohols, typically have pleasant odors and are used in various applications including food flavoring.

Nitrogen-Containing Functional Groups

• Amines: Organic bases, behave similarly to ammonia.
• Amides: Contain both carbonyl and amine features.

Isomerism in Organic Chemistry

• Isomers can exhibit different structural configurations, leading to distinct physical and chemical properties.
  • Increasing carbon count enhances potential isomer variety.

Chirality in Organic Chemistry

• Chirality: A molecule that presents as non-superimposable mirror images (enantiomers) often centers on a carbon atom bonded to four different substituents.
• Enantiomers are designated as R and S, and racemic mixtures contain equal amounts of both isomers.

Practice Problems

• Examples are given for naming compounds and identifying isomers based on molecular formulae.