Organic Chemistry: Hydrocarbons and Heteroatoms

Hydrocarbons

Saturated Hydrocarbons

• Definition: Each carbon is bonded to a maximum of 4 atoms, following the fundamental rule that carbon must always have 4 bonds. Saturated hydrocarbons, also known as alkanes, are composed entirely of single bonds between carbon and hydrogen atoms, maximizing the number of hydrogen atoms per carbon.

• Homologous Series: Described by the formula CnH{2n+2}, where each member differs from the next by CH_2. This series illustrates how alkane properties change incrementally with increasing carbon chain length.

• Alkanes:

  • Examples: Methane (CH4), Ethane (C2H6), Propane (C3H_8), etc.

  • General formula: CnH{2n+2}

  • Methane: Found in sheep and natural gas, it is the simplest alkane and a significant greenhouse gas.

  • Ethane: Found in pine trees; serves as a building block in chemical synthesis.

  • C3-4: Components of LPG (Liquefied Petroleum Gas), used as fuel in heating appliances and vehicles.

  • C5-12: Make up petrol (octane), the primary component of gasoline used in internal combustion engines.

  • C12-16: Constitute kerosene, commonly used as jet fuel and in lamps.

  • C15-18: Used in diesel oil, a fuel for diesel engines.

  • C18+: Found in lubricating oils and bitumen, used to reduce friction in engines and as a binder in asphalt.

  • Most are obtained by fractionation of crude oil (>C_3), a refining process that separates crude oil into its different components based on boiling points.

• Alkane Character:

  • All carbon atoms are tetrahedral with sp^3 hybridization, meaning each carbon is at the center of a tetrahedron with four equally spaced bonds.

  • Bond angles are approximately 109.5°, the ideal tetrahedral angle that minimizes electron repulsion.

  • C-C single bonds involve end-to-end overlap between sp^3 hybrid orbitals on adjacent carbons, forming a sigma (\sigma) bond, which is a strong covalent bond.

• Silanes:

  • Formula: SinH{2n+2}

  • The periodic table explains the existence of silanes, as silicon is in the same group as carbon and can form similar compounds.

• Alkane Properties:

  • Nonpolar compounds due to the similar electronegativity values of carbon and hydrogen.

  • Low boiling points because they only exhibit weak London dispersion forces.

    • Alkanes with 1-4 carbons are gases at room temperature.

    • Alkanes with 5-17 carbons are liquids.

    • Alkanes with >18 carbons are waxy solids.

  • Low melting point and low density; they are less dense compared to many other organic compounds.

  • Melting point increases with molar mass as larger alkanes have greater intermolecular forces.

  • Average density is 0.7 g/mL for 1-10 carbons, causing them to float on water (e.g., petrol spills), which is a significant environmental concern.

• Aliphatic Group:

  • An alkane, alkene, or alkyne that is a substituent or a parent chain. Example: aliphatic alcohol = ethanol, where the parent is ethane, indicating the carbon chain is not aromatic.

• Cycloalkanes:

  • General formula: (CH2)n or CnH{2n}, formed by removing 2H and joining the ends of the alkane chain to form a ring.

  • All C-C single bonds in a ring, creating stable cyclic structures.

  • 5- and 6-membered rings are the most common due to minimal ring strain.

  • Cyclobutane (C4H8): “Cyclobutane pyrimidine dimers are predominant DNA lesions in whole human skin exposed to UVA radiation,” indicating its role in DNA damage from UV exposure.

  • Cyclopentane (C5H{10}): Large amounts are used industrially as a solvent and in the production of other chemicals. “GE launches new cyclopentane refrigerators 2011,” showcasing its use as an environmentally friendly blowing agent in insulation.

Unsaturated Compounds

• Contain double or triple bonds, reducing the number of hydrogen atoms compared to alkanes.

• Alkenes:

  • Formula: CnH{2n}

  • Contain a double bond, making them more reactive than alkanes.

• Cycloalkenes:

  • Formula: CnH{2n-2}

  • Contain a double bond and are cyclized, combining the properties of alkenes and cycloalkanes.

• Polyenes:

  • Contain multiple double bonds and are prevalent in natural pigments.

• Alkynes:

  • Formula: CnH{2n-2}

  • Contain a triple bond, making them highly reactive. They are also key components in organic synthesis.

  • Some gases are produced from crude oil during cracking processes.

• Alkenes Importance: Immense in biology & industry. They are crucial in polymer production and natural processes.

  • Ethene (ethylene): 20 x 10^6 tons/year; the largest production of any organic chemical, highlighting its industrial significance.

  • Uses: Production of polymers (feedstock), e.g., polyethylene, polystyrene; ripening of fruit, demonstrating its versatility in both industry and agriculture.

• Alkene Pheromones: Widely used in biology as communication signals between insects.

  • Muscalure: Housefly sex attractant - CH3(CH2)7CH=CH(CH2){12}CH3.

  • (Z)-9-Tetradecenyl acetate: Used by the bolas spider Mastophora cornigera to attract male prey species by mimicking the female sex pheromone of the prey species they exploit. This showcases the sophisticated use of pheromones in predation.

• Chemistry of Vision:

  • β-carotene (polyene): A photochemical mechanism of vision involves cis-trans (E/Z) isomerization at C11. The shape change is transferred to the opsin, into the lipid membrane, then to nerve cells and back to the brain. This process is fundamental to how we perceive light.

  • Retinol (Vitamin A) and Retinal are key components in the visual cycle, essential for light detection in the retina.

Alkene & Alkyne Character

• Alkenes:

  • One or more carbon-carbon double bonds (C=C), which are regions of high electron density and reactivity.

  • Bond angles around each sp^2 carbon atom in the double bond are ~120°, resulting in trigonal planar geometry at the carbon. This geometry influences the molecule's shape and reactivity.

  • The C=C double bonds involve end-to-end overlap between sp^2 hybrid orbitals (\sigma-bond) and side-to-side overlap between parallel p orbitals (\pi-bond) on adjacent carbons. The combination of sigma and pi bonds creates the double bond.

• Alkynes:

  • One or more carbon-carbon triple bonds (C≡C) are the defining feature.

  • Bond angles around each sp carbon atom in the triple bond are ~180°, resulting in linear geometry at the carbon. This linear structure is critical to alkyne properties.

  • The C≡C triple bonds involve end-to-end overlap between sp hybrid orbitals (\sigma-bonds) and side-to-side overlap between 2 x parallel p orbitals (2 x \pi-bonds) on adjacent carbons. This combination results in a strong and reactive triple bond.

Alkene / Alkyne Properties

• Physical properties of alkenes:

  • Nonpolar compounds due to the small difference in electronegativity between carbon and hydrogen.

  • Melting points, boiling points, and densities are very close to the related chain length alkanes, but slightly lower due to less efficient packing.

  • Ethene has a faint sweet smell; the rest are odorless, allowing for their detection in specific industrial applications.

  • Common names: ethylene = ethene, acetylene = ethyne (oxy-acetylene welding), which are important in industrial and chemical contexts.

Aromatic Compounds

• Contain very stable planar ring structures that look like (but are not) polyunsaturated, giving them unique chemical properties.

• They react differently to molecules with C=C, and theoretically, there are no localized double bonds but rather a delocalized \pi-system above and below the aromatic ring. This delocalization contributes to their stability.

• The simplest aromatic is benzene (C6H6).

• An aromatic is a cyclic, planar molecule drawn with alternating double and single bonds; there may be more than one ring (e.g., naphthalene). The alternating bonds and planarity are crucial for aromaticity.

• More Aromatic Compounds:

  • PAHs (polycyclic aromatic hydrocarbons): Benzo[α]pyrene is a potent carcinogen & mutagen found in cigarette smoke, incinerators, car exhausts, and barbecued steaks. These compounds are harmful environmental pollutants.

  • Phenylalanine (amino acid): One of several aromatic amino acids essential in protein synthesis. It is a building block of proteins and plays a key role in biological processes.

  • Phenol: Aromatic alcohol, used in antiseptic and disinfectant applications.

• Aromatics Character:

  • Orbital overlap model in an arene (aromatic) compound: All parent skeletal carbons are sp^2, with end-to-end overlap between sp^2 hybrid orbitals (\sigma-bonds) creating a cyclic carbon framework, and side-to-side overlap of p orbitals creating a delocalized \pi cloud. This model explains the stability and unique reactivity of aromatic compounds.

  • Aryl group: An arene that is a substituent, showing how aromatic rings can attach to other molecules.

  • Carbon-carbon bond lengths: C-C is 1.54 Å; C=C is 1.33 Å. The actual bond length in benzene is intermediate between these values due to electron delocalization, approximately 1.39 Å.

  • Kekulé structures represent the classical depiction of alternating single and double bonds, but these do not accurately reflect electron delocalization.

Aromatic Properties

• Physical properties of aromatics (arenes):

  • Nonpolar compounds due to the symmetrical distribution of electrons in the ring.

  • Moderately low melting points, boiling points, & densities, influenced by the ring structure and intermolecular forces.

  • Slightly different intermolecular forces to cycloalkenes influence the melting point in particular.

  • Benzene has a boiling point of 80 °C and a melting point of 6 °C, key data for identifying and using benzene in chemical processes.

  • Benzene is soluble in organic solvents and slightly soluble in water, relevant for its use in industrial applications.

  • Benzene is a good organic solvent due to its nonpolar nature.

  • Cyclohexa-1,3-diene has a boiling point of 81 °C and a melting point of -89 °C, demonstrating how its non-aromatic structure affects its physical properties compared to benzene.

• Naming Hydrocarbons:

  • Alkenes and Alkynes: Monosubstituted benzenes are named as benzene derivatives, except for certain common names retained by the IUPAC system, indicating the standardized nomenclature for organic compounds.

Compounds with Heteroatoms

• Bond polarity: Polar sigma bond with partial positive (\delta^+) and partial negative (\delta^-) charges, where X_3C-Z and Z is an electronegative atom (heteroatom) in the functional group (e.g., O, N, halogen). This polarity significantly affects the compound's reactivity and properties.

• Haloalkanes, Alcohols, Ethers, and Amines: Carbon-Heteroatom Single Bonds.

• Haloalkanes:

  • Formula: R-X, where X is a halogen atom covalently bonded to an sp^3 hybridized carbon atom (X = F, Cl, Br, or I). These compounds are versatile in industry and synthesis.

  • Uses:

    • Solvents: 1,1,1-trichloroethane (CH3CCl3).

    • Refrigerants: HCFCs (e.g., HCFC-123, C2HCl2F_3).

    • Anesthetics: haloethane (CF_3CHClBr).

  • Environmental problems:

    • CHCl3, CCl4: Cumulative toxicity, carcinogens. Addressing these issues is important for public health.

    • CFCs (e.g., Freon 12, CF2Cl2): Ozone depletion (e.g., 1,1-dichloro-1,1-difluoromethane). Their phase-out is vital for environmental protection.

• Haloalkanes: Haloalkenes Naming:

  • Naming haloalkanes (R-X), also called alkyl halides:

    • Locate and number the parent chain, numbering to give substituent(s) the lowest number(s). This ensures proper identification and communication among chemists.

    • Use fluoro-, chloro-, bromo-, and iodo- for halogen substituents, listing them in alphabetical order. This follows IUPAC naming conventions.

    • Use a number proceeding the name of the halogen to locate it on the parent chain.

    • In haloalkenes, the location of the double bond determines numbering of the parent hydrocarbon, prioritizing the functional group.

• Alcohols:

  • Formula: R-CH_2-OH, where a hydroxyl atom is covalently bonded to a tetrahedral sp^3 carbon atom. Alcohols are widely used as solvents, antiseptics, and chemical intermediates.

  • Examples:

    • Methanol (CH_3OH): Emitted by grass (methyl alcohol). It is also a fuel additive and industrial solvent.

    • Ethanol (C2H5OH): Produced from yeast + sugar + water (ethyl alcohol) = first “chemical synthesis”. It is a widely used solvent, antiseptic, and fuel.

    • Ethylene glycol: Antifreeze, used in automotive cooling systems.

    • Glycerol: Backbone of fats and vegetable oils, used in cosmetics, pharmaceuticals, and food.

  • Silanols (silicon alcohols): E.g., H_3SiOH, analogs of alcohols with silicon replacing carbon.

• Alcohols: Character:

  • The hydroxyl group (OH) is bonded to an sp^3 hybridized C atom.

  • The hydroxyl O atom is sp^3 hybridized, influencing its bonding and reactivity.

  • Two O sp^3 hybrid orbitals form σ bonds to C & H, while the remaining two contain lone pairs.

  • Alcohols are classified as 1°, 2°, or 3° based on the number of alkyl groups replacing H atoms at C:

    • Primary (1°) alcohols (RCH2OH): E.g., propan-1-ol (CH3CH2CH2OH).

    • Secondary (2°) alcohols (R2CHOH): E.g., propan-2-ol (CH3)_2CHOH (isopropanol).

    • Tertiary (3°) alcohols (R3COH): E.g., 2-methylpropan-2-ol (CH3)_3COH (tert-butanol).

• Alcohols: Properties:

  • Physical properties:

    • Both the C-O and O-H bonds of an alcohol are polar covalent, leading to hydrogen bonding.

    • Alcohols are polar molecules (e.g., methanol), affecting their physical properties.

    • Alcohols have higher boiling points and water solubility than hydrocarbons due to hydrogen bonding.

    • Alcohols associate in the liquid state by hydrogen bonding, enhancing intermolecular interactions.

    • High water solubility is due to hydrogen bonding between alcohol and water molecules.

    • Electronegativity values: O = 3.5, C = 2.5, H = 2.1. These values explain the bond polarity.

Ethers

• Ethers: Character & Properties

  • Oxygen is sp^3 hybridized with bond angles of approximately 109.5° (tetrahedral) in R-O-R.

  • Ether linkage: C-O-C is the defining structure of ethers.

  • Physical properties:

    • Ethers are low-moderately polar compounds.

    • Ethers have lower boiling points than alcohols of similar molar mass because they cannot form hydrogen bonds with themselves.

    • Ethers are insoluble in water due to limited hydrogen bonding.

    • Only weak dipole-dipole forces of attraction exist between ether molecules.

    • Electronegativity values: O = 3.5, C = 2.5, H = 2.1.

• Naming Alcohols / Phenols

  • Alcohols: The parent is the longest chain of carbon atoms containing the –OH group. Change the suffix of the parent from –e to –ol, use a number to show the location of the –OH group. Name and number substituents and list them in alphabetical order, in accordance with IUPAC rules.

  • Phenols: Phenols contain an OH group bonded to a benzene ring (PhOH, C6H5OH). Substituted phenols are named as derivatives of phenol (phenol OH carbon = C1).

• Naming Ethers

  • Named as alkoxyalkanes. The -OR group bonded to the parent alkane (longest chain) is named as an alkoxy (RO-) group (methoxy, ethoxy, etc.). Common names for ethers are derived by listing the alkyl groups bonded to oxygen in alphabetical order and adding ether as a separate word (e.g., ethyl methyl ether).

Hydrogen Bonding in Alcohols and Amines

• Lone pair(s) of electrons are available on O (alcohols, ethers) or N (amines).

• Due to the polarity of the O-H or N-H bonds, the lone pair(s) may interact with electron-deficient H’s on adjacent molecules to form hydrogen bonds. These bonds are crucial for intermolecular forces.

• This means small molecules with these functional groups are water-soluble, due to their ability to form hydrogen bonds with water.

Amines

• Amines are derivatives of ammonia NH_3. They are crucial in biological systems and chemical synthesis.

• Amines are classified as 1°, 2°, or 3° based on the number of alkyl groups replacing hydrogen atoms of NH_3, affecting their reactivity and basicity.

• Amines have many therapeutic and ‘other’ uses as drugs (e.g., antihistamines, painkillers); they interact with biological systems due to their basic nature.

• Weak bases: RNH2 + H2O \rightleftharpoons RNH_3^+ + OH^-; this equilibrium is crucial for understanding their reactivity.

  • Examples: RNH2, R2NH, R_3N

  • Common names provide an alternative nomenclature.

• Amine Examples:

  • The first 4 are constitutional isomers of C4H{11}N, differing in atom connectivity. Common names are in blue. Isomers show how different structures can have the same formula.

  • Aniline is an aromatic amine. Its aromatic structure impacts its chemical behavior.

  • The last 3 are important heterocyclic aromatic amines, found in DNA and other biomolecules. Their ring structures contain nitrogen atoms.

• Naming Amines:

  • Replace the suffix -e of the parent alkane with –amine, following IUPAC nomenclature.

  • 2° or 3° amines are named as N-alkylated 1° amines. Common names list the alkyl groups bonded to nitrogen in alphabetical order, ending in the suffix –amine.

• Amines: Character & Properties:

  • Physical properties:

    • Amines are polar compounds; both 1° and 2° amines form intermolecular hydrogen bonds.

    • N-H- - - -N hydrogen bonds are weaker than O-H- - - -O hydrogen bonds because the difference in electronegativity between N and H is less than that between O and H. This difference influences their physical properties.

    • Much higher boiling points than hydrocarbons due to hydrogen bonding.

    • Amines tend to be smelly: low M.W. amines (gases) have strong odors.

      • Trimethylamine: Fish odor.

      • Butane-1,4-diamine: “Putrescene”.

    • Electronegativity values: O = 3.5, N = 3.0, C = 2.5, H = 2.1. These values help to explain their polar nature.