Organic Chemistry

Organic Chemistry: Introduction to Carbon Compounds

Early Synthesis and Significance

  • In 1828, Friedrich Wöhler synthesized urea, an organic compound found in cells, using inorganic reactants.
  • This was the first instance of producing an organic compound in a lab, disproving the need for a "vital force."
  • This discovery revolutionized carbon chemistry, demonstrating that compounds from living organisms could be created artificially.

Carbon Compounds: Abundance and Origin

  • Carbon compounds are incredibly diverse and numerous, with millions known and hundreds of thousands newly synthesized annually.
  • They primarily consist of carbon combined with elements like hydrogen, oxygen, nitrogen, sulfur, phosphorus, and halogens.
  • Due to their vast number, an entire branch of chemistry is dedicated to carbon compounds.

General Properties of Carbon Compounds

  • Carbon compounds are generally nonelectrolytes or weak electrolytes.
  • They tend to have low melting points.
  • Compounds made of only carbon and hydrogen are typically nonpolar and insoluble in water.
  • Other carbon compound classes have varying degrees of solubility.

Sources of Carbon Compounds

  • Originally, carbon compounds were isolated from coal and petroleum.
  • The largest carbon store is in carbonates and minerals, vastly exceeding fossil fuels and forests.

Formation of Coal

  • Millions of years ago, vegetation grew rapidly and formed thick deposits.
  • Pressure and heat transformed buried plant material into coal, composed mainly of carbon, with other elements incorporated.
  • Destructive distillation, heating coal without air, releases various carbon compounds.
  • Coke, a residue, replaced charcoal in iron making.
  • Coal tar was separated into over 200 carbon compounds for direct use or lab treatment.

Formation of Petroleum and Natural Gas

  • Microscopic marine organisms died and settled on the seafloor.
  • Decomposition by microbes produced methane (CH4).
  • High temperatures and pressures altered molecular structures into crude oil deposits in porous sandstone.

Organic Chemistry Definition

  • Initially, organic chemistry was named due to the belief that carbon compounds came only from living organisms.
  • The name persists even though organic chemicals are now synthesized in labs.

Bonding Behavior of Carbon

  • Carbon has four valence electrons, allowing it to form four covalent bonds, including double and triple bonds.
  • Carbon atoms can link to form chains of variable length and rings.
  • The arrangement of atoms affects a substance's properties; different arrangements yield unique substances.
  • For example, C30H62 can form about four billion possible compounds.

Hydrocarbons and Derivatives

  • Hydrocarbons contain only carbon and hydrogen.
  • Derivatives of hydrocarbons result when hydrogen atoms are replaced by other atoms.
  • Hydrocarbons serve as building blocks for complex organic molecules.
  • Common sources are coal, petroleum, natural gas, and certain trees.

Carbon Chains and Backbones

  • The longest carbon chain is called the carbon backbone or skeleton.
  • Structural diagrams often show only the carbon skeleton, with hydrogen atoms implied.
  • Hydrocarbons can be straight-chain (unbranched) or branched.

Saturated vs. Unsaturated Hydrocarbons

  • Hydrocarbons can have single, double, or triple bonds.
  • Reactivity depends on the number and type of multiple bonds.
  • Saturated hydrocarbons have only single carbon-carbon bonds and are relatively stable.
  • Unsaturated hydrocarbons have at least one double or triple carbon-carbon bond and can incorporate additional atoms.
  • Carbon-carbon bonds are described as saturated or unsaturated based on whether they are single, double, or triple bonds respectively.

Alkanes: Saturated Hydrocarbons

  • Alkanes are straight- or branched-chain saturated hydrocarbons with only single bonds.
  • They are obtained through fractional distillation of petroleum.
  • Alkanes have relatively low boiling points due to weak intermolecular attractions (London forces).
  • Boiling points increase with the number of atoms due to increased London forces.

Methane and Other Alkanes

  • Methane (CH4) is the simplest alkane, produced during anaerobic decomposition and is a major component of natural gas.
  • Some believe methane existed in Earth's early atmosphere and played a role in the evolution of life.
  • Ethane (C2H6) contains one carbon-carbon single bond and six carbon-hydrogen bonds and is also in natural gas.
  • Propane (C3H8) is in natural gas and used as fuel in portable stoves.

Homologous Series

  • A homologous series consists of compounds that differ by the addition of the same structural unit (-CH2-).
  • Alkanes are an example of a homologous series.
  • The general formula for alkanes is CnH2n+2, where n is the number of carbon atoms.

Naming Hydrocarbons (IUPAC Nomenclature)

  • The International Union of Pure and Applied Chemistry (IUPAC) developed a systematic method for naming organic compounds.
  • Unbranched alkanes are named based on the number of carbons in the longest straight chain (Greek prefixes + "ane").
  • For branched alkanes, the smaller portion or branch is named as an alkyl group by replacing "ane" with "yl."
  • Numbers indicate the position of the branch on the main chain, using the lowest possible number.

Structural Isomers

  • Structural isomers are compounds with the same chemical formula but different arrangements of atoms.
  • Isomers have different physical properties (melting points, boiling points, solubilities).
  • The IUPAC naming system distinguishes between isomers.
  • Example: 2,4-dimethylheptane (longest chain is heptane, two methyl groups at positions 2 and 4).

Alkenes: Hydrocarbons with Double Bonds

  • Alkenes are straight- or branch-chained hydrocarbons containing at least one double carbon-carbon bond.
  • They are produced by cracking larger alkanes at high temperatures with catalysts.
  • The general formula is CnH2n.
  • Alkene names are similar to alkanes, but the ending is changed from "ane" to "ene."
  • Structural isomers exist based on the location of the double bond, which is indicated by the lowest number of the carbon atom involved in the bond.

Reactivity of Alkenes

  • Alkenes are more reactive than alkanes due to the presence of a double bond.
  • One bond of the double bond is more easily broken, making alkenes more susceptible to chemical reagents.

Ethene (Ethylene)

  • Ethene (C2H4) is the simplest alkene, a gas with a slightly sweet odor.
  • Plants produce ethene naturally.
  • It's produced during hydrocarbon refining and found in petroleum and natural gas.
  • Ethene is crucial in the chemical industry for producing ethyl alcohol, solvents, plastics, gasoline additives, antifreeze, and detergents.

Alkynes: Hydrocarbons with Triple Bonds

  • Alkynes contain a triple carbon-carbon bond.
  • They are unsaturated and reactive, with the general formula CnH2n-2.
  • Names are based on the parent alkane with the ending "yne."

Naming and Isomers of Alkynes

  • The location of the triple bond is indicated by the lowest number of the carbon involved in the bond.
  • Structural isomers exist based on the triple bond's position.

Ethyne (Acetylene)

  • Ethyne (C2H2), commonly known as acetylene, is the simplest and most common alkyne.
  • It's an explosive gas used in oxyacetylene torches for cutting and welding steel.

Cycloalkanes: Saturated Hydrocarbon Rings

  • Cycloalkanes are saturated hydrocarbons in the form of a ring.
  • They can have three, four, five or more carbon atoms.
  • Named by using the prefix "cyclo-" before the corresponding alkane name (e.g., cyclopentane).
  • Geometric shapes represent their structures, with each point being a carbon atom bonded to two hydrogen atoms.

Stability and Reactivity of Cycloalkanes

  • Three- and four-carbon rings (cyclopropane and cyclobutane) are less stable due to bond angle strain.
  • The bond angles are compressed from the ideal 109.5°.
  • These compounds tend to undergo reactions that relieve strain by opening the ring.

Cycloalkenes

  • Cyclic structures can also exist among alkenes.
  • Unsaturated cyclic hydrocarbons are often more reactive than their saturated or straight-chain counterparts and can be unstable.

Aromatic Hydrocarbons: Benzene and its Derivatives

  • Aromatic hydrocarbons possess distinctive fragrances and were initially obtained from coal tar distillation.
  • Benzene (C6H6) is the simplest aromatic compound, with a basic structure of one or more six-carbon rings.

Structure and Bonding in Benzene

  • Benzene's atoms lie in a plane, with 120° angles between bonds formed by a carbon atom.
  • The carbon-carbon bond distance in benzene is intermediate between a single and double bond.

Delocalized Electrons in Benzene

  • Electrons in benzene are shared equally around the ring and are not associated with any one carbon atom (delocalized).
  • The structure of benzene is often represented as a hexagon with a circle inside to denote delocalized electrons.

Uses of Benzene

  • Benzene serves as a starting material for thousands of compounds.
  • Benzene derivatives are used to produce plastics, synthetic fibers, dyes, medicines, anesthetics, synthetic rubber, food additives, paints, and explosives.

Petroleum Refining: Fractional Distillation

  • Petroleum is a mixture of hydrocarbons that must be separated before use.
  • The main components are alkanes and cycloalkanes, along with other substances.
  • Fractional distillation separates hydrocarbons based on condensation temperatures.
  • Crude oil is heated, vaporized, and then rises in a fractionating tower, condensing at different levels.
  • Materials are drawn off at different heights corresponding to condensation temperatures.

Further Processing of Petroleum Fractions

  • Long-chain alkanes undergo cracking to produce better quality gasolines and important alkenes like ethene.
  • Other groups of materials are subjected to further refining, separation, and purification.

Functional Groups in Organic Molecules

  • Characteristics of organic molecules depend on their composition and arrangement of atoms.
  • A functional group is an atom, group of atoms, or organization of bonds that determines specific properties of a molecule.
  • Functional groups are generally the most reactive portion of a molecule.

Common Functional Groups

  • Include alcohols (-OH), ethers (R-O-R'), aldehydes (RCHO), ketones (RCOR'), organic acids (-COOH), esters (RCOOR'), and amines (RNH2).
  • R represents the rest of the molecule to which the functional group is attached.
  • Organic compounds with the same functional group behave similarly in chemical reactions.

Alcohols

  • Contain the hydroxyl group (-OH) bonded to a carbon atom.
  • Named by replacing the "e" ending of the corresponding alkane with "ol".
  • The hydroxyl group is polar and can form hydrogen bonds, increasing water solubility.

Methanol and Ethanol

  • Methanol (CH3OH), or methyl alcohol, is used in synthesizing plastics and fibers.
  • Ethanol (CH3CH2OH), or ethyl alcohol, is produced by fermentation and used in alcoholic beverages, industrial processes, and medicines.

Polyhydric Alcohols

  • Alcohol molecules can have more than one hydroxyl group.
  • 1,2-ethanediol (ethylene glycol) is used as antifreeze and in the synthesis of polyester fabrics.

Ethers

  • Have an oxygen atom bonded between two hydrocarbon groups.
  • Exhibit little hydrogen bonding and have low boiling points.
  • Ethoxyethane (diethyl ether) was once used as an anesthetic and continues to be used as a solvent for oils and fats.

Aldehydes and Ketones

  • Both contain the carbonyl functional group (C=O).
  • In aldehydes, the carbonyl group is attached to a carbon atom with at least one hydrogen atom, written as RCHO.
  • In ketones, the carbonyl group is attached to a carbon atom bonded to two other carbon atoms, written as RCOR'.
  • Aldehydes are named by replacing the "e" ending of the parent alkane with "al" (e.g. methanal or formaldehyde).
  • Ketones are named by replacing the "e" ending of the parent alkane with "one" (e.g. propanone or acetone).

Organic Acids (Carboxylic Acids)

  • Contain the carboxyl functional group (-COOH) are polar.
  • They weakly dissociate the hydrogen atom from the carboxyl group.
  • Named by replacing the "e", ending of the parent alkane with "oic acid".
  • Ethanoic acid, or acetic acid, in diluted form (5% solution with water) is called vinegar.

Esters

  • Produced from a reaction between organic acids and alcohols (esterification).
  • The process is a reversible dehydration reaction in which the alcohol loses a hydrogen atom and the acid loses the -OH part of its carboxyl group.

Properties and Uses of Esters

  • Most esters possess distinctive aromas and flavors.
  • They're found naturally in fragrant foods and are used synthetically as perfume additives and artificial flavorings.

Amines

  • Organic compounds closely related to ammonia.
  • Organic R- groups can replace one, two, or three hydrogens in the ammonia molecule (methylamine, dimethylamine, trimethylamine).

Amides

  • Produced when an amine or ammonia is treated with an organic acid.
  • Contain a carbonyl group bonded to the nitrogen atom of an amine (amide linkage).

Properties of Amides

  • They form strong intermolecular bonds, linking amino acids in protein molecules.
  • They connect other molecular units to form larger molecules like Nylon.

Substitution Reactions

  • A hydrogen atom is replaced by another atom or group of atoms.
  • Ex: Methane reacting with chlorine gas to form chloromethane (CH4 + Cl2 \rightarrow CH_3Cl + HCl).
  • Further substitution can produce dichloromethane (CH2Cl2), trichloromethane (CHCl3), and tetrachloromethane (CCl4).

Displacement Reactions

  • Similar to substitution but involves functional groups.
  • Example: Chloromethane reacting with ammonia to produce methylamine (H3C-Cl + NH3 \rightarrow H3C-NH2 + HCl).

Addition Reactions

  • Carbon compound containing one or more double (or triple) bonds reacts with another substance breaking double or triple bonds.
  • Ex: Ethene reacting with water to produce ethanol (CH2=CH2 + H2O \rightarrow CH3CH_2OH).

Oxidation Reactions

  • Addition of oxygen or removal of hydrogen from a molecule.
  • Ex: Alcohol to aldehyde (R-CH2OH \rightarrow R-CHO + H2) using an oxidizing agent.

Reduction Reactions

  • Reverse of oxidation (removal of oxygen, addition of hydrogen).
  • Ex: Reducing aldehyde to alcohol.

Polymerization

  • The process by which extended chain structures are formed, producing giant molecular chains and high molecular weight.
  • Monomers: basic repeating units.
  • Polymers: molecules composed of repeating sequence of monomers.

Addition Polymerization

  • Bonding of monomers without elimination of atoms, accomplished by opening unsaturated bonds.
  • Ex: Polymerization of ethene to polyethylene:
    n(C2H4) \rightarrow (C2H4)_n

Condensation Polymerization

  • Formation of a polymer accompanied by the elimination of atoms.
  • Ex: Combination of amino acids to form polypeptide chains.
  • Water is eliminated when the amine end of one molecule joins with the acid end of another forming what is called a peptide bond.

Nylon Synthesis

  • Condensation polymerization between diamines and dicarboxylic acids.
  • Example:
    nH2N-(CH2)6-NH2 + nHOOC-(CH2)4-COOH
    into Nylon-66
  • Example of successful synthesis due to use of long reacting molecules rather than attempting to combine small ones.