orgchem lec

• Aliphatic Hydrocarbons


  Hydrocarbons based on chains of C atoms
• Types of Aliphatic Hydrocarbons

  • Alkanes with only single covalent bonds
  • Alkenes contain at least one C=C
  • Alkynes contain a C≡C
  • Alicyclic hydrocarbons
• Aliphatic


  (from Greek aleiphas, "fat") described hydrocarbons derived by chemical degradation of fats or oils
• Alicyclic hydrocarbons


  Compounds which contain ring of 3 or more carbon atoms which resemble aliphatic (acyclic) hydrocarbons in their properties
• Types of Alicyclic Hydrocarbons


  • Cycloalkanes
  • Cycloalkenes
  • Cycloalkynes
• Physicochemical properties of Aliphatic HC


  • Most are highly flammable and readily react with oxygen in combustion reactions
  • Liquid aliphatic hydrocarbons have high vapor pressure
  • Nonpolar aliphatic hydrocarbons have equal charge distribution across the molecule
• Pharmaceutical Uses of Hydrocarbons

  • They are the principal constituents of petroleum and natural gas, serving as fuels, lubricants, and raw materials for production of plastics, fibres, rubbers, solvents, explosives, and industrial chemicals
  • Many hydrocarbons occur in nature, e.g. as pigments called carotenes in carrots and green leaves
  • More than 98 percent of natural crude rubber is a hydrocarbon polymer
• Alkanes


  Aliphatic hydrocarbons with only single covalent bonds
• Physical Properties of Alkanes


  • Alkanes and cycloalkanes are nonpolar substances, with attractive forces between molecules dictated by weak London forces
  • Alkanes have low boiling points compared to polar molecules of comparable molecular weight
  • Boiling points of alkanes increase with increasing number of carbons
  • Simplest alkanes have C atoms bonded in a straight chain (normal alkanes)
• Chemical Properties of Alkanes

  • Alkanes burn in air to produce carbon dioxide and water, releasing heat
• Pharmaceutical Uses of Alkanes


  They are used as solvents, heating oils, fuels, in fat synthesis, in the synthesis of fatty acids by air oxidation, in the manufacture of albumen, and in the transformation to olefins
• Condensed Formula


  A line-angle formula where carbon atoms are implied at the corners and ends of lines, and each carbon atom is understood to be attached to enough hydrogen atoms to give each carbon atom four bonds
• Structural Formula


  Diagrams representing alkanes that show the structure of the molecule
• Condensed Structural Formula


  Lists the formula of each C atom in the backbone of the molecule
• Alkenes


  Hydrocarbons containing a carbon-carbon double bond, sometimes called olefins
• Chemical Properties of Alkenes

  • Valued mainly for addition reactions, in which one of the bonds in the double bond is broken
  • Use in pharmaceutical preparations is very restricted due to toxicity
  • More commonly used in industry as raw materials for manufacturing alcohols and aldehydes
  • Abundant in nature, e.g. ethylene (plant hormone), essential oils, lycopene (plant pigment)
• Physical Properties of Alkenes

  Alkenes have similar physical properties to alkanes and alkynes
• Naming Alkenes


  1. Number the carbons in the chain so that double bond carbons have lowest possible numbers

  2. Write the full name

  3. Number substituents according to position in chain and alphabetically

  4. Rings have cyclo as prefix
• Naming Alkenes - Examples

  1. The longest chain is a pentene, with three methyl groups attached to C3 and C4. The compound is 3,4,4-trimethyl-1-pentene.

  2. The longest chain is a hexane, with a methyl group attached to C3. The compound is 3-methyl-3-hexene.
• Chemical Reactions of Alkenes

  1. Hydrogenation: reaction with hydrogen (H2) in the presence of a catalyst such as nickel (Ni) or platinum (Pt)

  2. Halogenation: addition of halogens

  3. Hydration: requires a catalyst, usually a strong acid like sulfuric acid (H2SO4)
• General Reaction Mechanism of Electrophilic Addition

  1. Attack on electrophile, such as HBr, by π bond of alkene

  2. Produces carbocation and bromide ion

  3. Carbocation is an electrophile, reacting with a nucleophilic bromide ion
• Markovnikov's Rule


  In electrophilic addition reactions, unsymmetrical substituted alkenes give a single product. H attaches to the carbon with fewer alkyl substituents, and X attaches to the carbon with more alkyl substituents.
• Regiospecific


  Reaction in which only one of two possible orientations of an addition occur
• Calculating Degree of Unsaturation or Index of Hydrogen Deficiency


  1. Alkenes have fewer hydrogens than alkanes with the same number of carbons

  2. Degree of unsaturation = 2n + 2 - h, where n is the number of C atoms and h is the number of H atoms
• Alkynes


  Hydrocarbon molecules with one or more triple bonds, CnH2n-2
• Properties of Alkynes


  • Each triple bond reduces the number of hydrogen atoms by 4
  • Alkynes are more reactive than alkanes
  • Alkynes have physical properties similar to alkanes and alkenes, but slightly higher boiling points
• Polyynes


  Linear carbon chains of sp-hybridized carbon atoms
• Ethyne


  The simplest member of the alkyne series, C2H2, commonly called acetylene
• Naming Alkynes


  1. General hydrocarbon rules apply

  2. Suffix -yne indicates an alkyne

  3. Number of the first alkyne carbon in the chain is used to indicate the position of the triple bond
• Naming Alkynes - Examples

  • 2,5-Dimethyl-3-hexyne
  • 3,3-Dimethyl-1-butyne
• Addition Reactions of Electrophiles with Alkynes

  Regiochemistry follows Markovnikov's rule
• Reactions of Alkynes


  Addition of HX and X2 forms trans addition products
• Mercury (II)-catalyzed Hydration of Alkynes

  1. Alkynes undergo hydration readily in the presence of mercury (II) sulfate catalyst

  2. Reaction occurs with Markovnikov chemistry

  3. Enol intermediate rearranges into a ketone through keto-enol tautomerism
• Keto-Enol Tautomerism


  Isomeric compounds that can rapidly interconvert by the movement of a proton
• Hydration of Unsymmetrical Alkynes


  Results in a mixture of both possible ketones
• 2,5-Dimethyl-3-hexyne


  1. Alkyne compound
• 3,3-Dimethyl-1-butyne


  1. Alkyne compound
• Addition reactions of electrophiles with alkynes are similar to those with alkenes
• Regiochemistry


  According to Markovnikov's rule
• Reactions of Alkynes: Addition of HX and X

  1. Addition products are formed when bromine and chlorine are added, resulting in trans stereochemistry

  2. Regiochemistry according to Markovnikov's rule