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Properties of Alcohols

Alcohols are one of the most important molecules in organic chemistry. They can be prepared from many different types of compounds and they can be converted into many different types of compounds. Alcohols are molecules containing the hydroxy functional group (-OH) that is bonded to a carbon atom of an alkyl or substituted alkyl group. The hydroxy functional group strongly contributes to the physical properties of alcohols.

Structure of Alcohols

The structure of an alcohol resembles that of water. With both alcohol and water, the bond angles reflect the effect of electron repulsion and increasing steric bulk of the substituents on the central oxygen. The electronegativity of oxygen contributes to the unsymmetrical distribution of charge, creating a partial positive charge on the hydrogen and a partial negative charge on the oxygen. This uneven distribution of electron density in the O-H bond creates a dipole. Also, because of the high electronegativity of oxygen relative to that of carbon, the O-H bond is shorter (1.41 Å for C-O vs. 1.51 Å for C-C) and stronger (ΔHoO-H = 104 kcal mol-1 for C-O vs. ΔHoC-H = 98 kcal mol-1 for C-C).

 

water.jpg     methanol.jpg

 

                   Water                                               Methanol

Hydrogen Bonding and Solubility

The physical properties of alcohols are influenced by the hydrogen bonding ability of the -OH group. The -OH groups can hydrogen bond with one another and with other molecules. 

hydrogen bonding of methanol.jpg

Hydrogen Bonding of Methanol

Hydrogen bonding raises the boiling point of alcohols. This is due to the combined strength of so many hydrogen bonds forming between oxygen atoms of one alcohol molecule and the hydroxy H atoms of another. The longer the carbon chain in an alcohol is, the lower the solubility in polar solvents and the higher the solubility in nonpolar solvents.

 

Physical Properties of Alchols and Selected Analogous Haloalkanes and Alkanes
Compound IUPAC Name Common Name Melting Poing (oC) Boiling Point (oC) Solubility in H2O at 23oC
 CH3OH  Methanol  Methyl alcohol  -97.8  65.0  Infinite
 CH3Cl  Chloromethane  Methyl chloride  -97.7  -24.2  0.74 g/100 mL
 CH4  Methane    -182.5  -161.7  3.5 mL (gas)/ 100 mL
 CH3CH2OH  Ethanol  Ethyl alcohol  -114.7  78.5  Infinite
 CH3CH2Cl  Chloroethane  Ethyl chloride  -136.4  12.3  0.447 g/100 mL
 CH3CH3  Ethane    -183.3  -88.6  4.7 mL (gas)/ 100 mL
 CH3CH2CH2OH  1-Propanol  Propyl alcohol  -126.5  97.4  Infinite
 CH3CH2CH3  Propane    -187.7  -42.1  6.5 mL (gas)/ 100 mL
 CH3CH2CH2CH2OH  1-Butanol  Butyl alcohol  -89.5  117.3  8.0 g/100 mL
 CH3(CH2)4OH  1-Pentanol  Pentyl alcohol  -79  138  2.2 g/100 mL

This table shows that alcohols (in red) have higher boiling points and greater solubility in H2O than haloalkanes and alkanes with the same number of carbons. It also shows that the boiling point of alcohols increase with the number of carbon atoms.

References

  1. Schore, Neil E. and Vollhardt, K. Peter C. Organic Chemistry: Structure and Function. New York: Bleyer, Brennan, 2007.
  2. Allen, Frank; Kennard. Olga; Watson, David G.; Brammer, Lee; Orpen, Guy; Taylor, Robin, J. Chem Soc. Perkin Trans II, 1987,S1-S19.

Problems

Arrange according to increasing boiling point. (start with lowest boiling point)

  1. CH4, CH3OH, CH3CH3
  2. CH3CH2Cl, CH3CH2CH2OH, CH3CH2CH3
  3. CH3CH2OH, CH3CH2Cl, CH4

Arrange according to increasing solubility (start with lowest solubility)

  1. CH4, CH3OH, CH3CH3
  2. CH4, CH3CH2Cl, CH3CH2OH,
  3. CH3CH2Cl, CH3CH2CH2OH, CH3CH2CH3

Answers

  1. CH4, CH3CH3, CH3OH  
  2. CH3CH2CH3, CH3CH2Cl, CH3CH2CH2OH
  3. CH3CH2Cl, CH4, CH3CH2OH
  4. CH3CH3, CH4, CH3OH
  5. CH3CH2Cl, CH4, CH3CH2OH,
  6. CH3CH2Cl, CH3CH2CH3, CH3CH2CH2OH

Contributors

  • Jaspreet Dheri
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Last Modified
10:20, 26 Feb 2014

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