Alkenes and alkynes

• Primary alcohols have one carbon atom attached to the OH group, secondary alcohols have two carbons atoms attached to the OH group, and tertiary alcohols have three or more carbons atoms attached to the OH group.
• Alkenes


  Contain a carbon-carbon double bond
• Alkenes


  • Exist in all three phases (solid, liquid, gas) at room temperature
  • Have melting and boiling points similar to alkanes
  • Cis alkenes have lower melting points than trans isomers
  • Display a weak dipole-dipole interaction due to the electron-attracting sp²carbon
• Alkenes


  A family of hydrocarbons containing a carbon-carbon double bond
• First two alkenes


  • ethene (C2H4)
  • propene (C3H6)
• Alkenes with 4 or more carbon atoms show structural isomerism
• Physical state of alkenes

  • Ethene, propene, butene are colorless gases
  • Pentene, hexene, heptene are liquids
  • 15 carbons or more are solids
• Density of alkenes


  • Lighter than water
  • Insoluble in water due to non-polar characteristics
  • Soluble in non-polar solvents
• Boiling points of alkenes

  • Similar to alkanes with the same number of carbon atoms
  • Depend on molecular mass (chain length), higher mass = higher boiling point
• Intermolecular forces of alkenes get stronger with increase in the size of the molecules
• Melting points of alkenes

  • Depend on the packaging of the molecules
  • Cis isomers have lower melting points than trans isomers due to U-bending shape
• Polarity of alkenes


  • The sp² carbon is more electron-withdrawing than sp³, creating a weak dipole
  • In trans-substituted alkenes the dipoles cancel out, in cis-substituted there is a net dipole
• Naming alkenes


  • The parent chain must include both carbon atoms of the double bond
  • The position of the double bond is indicated with a number
  • The -ane ending is replaced with -ene
• IUPAC rules for naming alkenes and cycloalkenes


  • The ene suffix indicates an alkene or cycloalkene

  2. The longest chain must include both carbon atoms of the double bond

  3. The chain is numbered from the end nearest the double bond

  4. The smaller of the two double bond carbon numbers is used

  5. In cycloalkenes the double bond is at C1 and C2

  6. Substituent groups containing double bonds are vinyl and allyl
• Cycloalkenes


  The double bond is assumed to be at C1 and C2, its location is not required in the name

  The numbering direction is chosen to give the substituents the lowest numbers
• Naming cycloalkenes


  • 3-Methylcyclopentene

  1. isopropyl-1-methylcyclohexene

  1,5-Dimethylcyclohexene

  1,3-Cyclohexadiene
• Common names of alkene fragments

  Vinyl group: H2C=CH-

  Allyl group: H2C=CH-CH2-
• ve-


  Rather than six-
• 1,4-pentadiene


  The second and fourth carbons are both substituted, so the numbering begins at the end nearest the alphabetically first-cited substituent (the ethyl group)
• Cycloalkenes


  • There are no chain ends, so the double bond is assumed to be numbered C1 and C2 and its location number is not required in the name
  • The direction of the numbering is determined by which will give the substituent closest to the double bond the lowest number
  • If multiple double bonds are present, it may be necessary to include their location numbers in the name
• Cycloalkenes


  • 3-Methylcyclopentene
  • 4-isopropyl-1-methylcyclohexene
  • 1,5-Dimethylcyclohexene
  • 1,3-Cyclohexadiene
• Common Names of Alkene Fragments

  • Methylene Group
  • Vinyl Group
  • Allyl Group
• In addition, the common name some small alkene compounds are still accepted by IUPAC. It is important to be able to identify them.
• Common Names


  • Ethylene
  • Propylene
  • Isobutylene
  • Isoprene
• Alkynes


  • Organic molecules with carbon-carbon triple bonds
  • Unsaturated hydrocarbons with the empirical formula of CnH2n-2
  • The simplest alkyne is ethyne which has the common name acetylene
• Alkynes


  • Generally nonpolar molecules with little solubility in polar solvents, such as water
  • Soluble in nonpolar solvents, such as ether and acetone
  • Tend to be gases if four or fewer carbon atoms
• Substituted alkynes


  Have small dipole moments due to differences in electronegativity between the triple-bonded carbon atoms, which are sp hybridized, and the single-bonded carbon atoms, which are sp hybridized
• Chemical compounds that have triple and double bonds in their chemical structures are referred to as unsaturated
• Alkynes


  • The triple bond contributes to the nonpolar bonding strength, linear structure, and acidity
• Solubility of Alkynes


  • Nonpolar, slightly soluble in polar solvents, insoluble in water
  • Soluble in organic solvents like benzene and carbon tetrachloride
• Boiling Points of Alkynes

  Slightly higher than alkanes and alkenes due to more polarizable electrons and stronger London dispersion forces
• Naming Alkynes


  • Use the suffix -yne
  • If a molecule contains both a double and a triple bond, the carbon chain is numbered so that the first multiple bond gets a lower number
  • If both bonds can be assigned the same number, the double bond takes precedence
• First ten carbon straight chain alkynes

  • Ethyne
  • Propyne
  • 1-Butyne
  • 1-Pentyne
  • 1-Hexyne
  • 1-Heptyne
  • 1-Octyne
  • 1-Nontyne
  • 1-Decyne
• IUPAC Rules for Alkyne Nomenclature

  • The yne suffix indicates an alkyne or cycloalkyne
  • The longest chain chosen for the root name must include both carbon atoms of the triple bond
  • The root chain must be numbered from the end nearest a triple bond carbon atom
  • The smaller of the two numbers designating the carbon atoms of the triple bond is used as the triple bond locator
  • If several multiple bonds are present, each must be assigned a locator number
  • Double bonds precede triple bonds in the IUPAC name, but the chain is numbered from the end nearest a multiple bond, regardless of its nature
  • In simple cycloalkynes the triple bond carbons are assigned ring locations #1 and #2
• Alkyne Fragments


  • Ethynyl group
  • Propargyl group
• Alkyne Compounds


  • 1,3-hexadien-5-yne
  • 3-penten-1-yne
  • 1-cyclobutyl-3-methyl-1-butyne
  • 4-vinyl-1-hepten-5-yne