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قديم 18-05-2004, 11:48 PM
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تاريخ الانتساب: 12 2003
المكان: ارض الرباط
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Physical Properties of Alcohols and Ethers
Boiling Points
The boiling points of alcohols are in general much higher than comparably sized ethers. That can be explained by the extra intermolecular forces between the alcohol molecules due to the hydrogen bonding. There is no such hydrogen bonding between the ether molecules because all the hydrogen atoms are not bonded to an Oxygen or a Nitrogen. Since there is not any possibility of Hydrogen bonding the forces between the ether molecules are much weaker and can be much more easily vaporized. As an example, the boiling point of ethanol, CH3-CH2-O-H, is 78 degrees Celsius. The boiling point of the constitutional isomer dimethyl ether, CH3-O-CH3, is only -24 degrees Celsius, more than 100 degrees lower!!! Since the size of the molecules of the two compounds are approximately the same, this dramatic difference can only be explained by the presence of hydrogen bonding with the alcohol. Branching will tend to decrease the boiling point since a more symmetrical molecule will have less London Dispersion forces and with more hydrocarbon branching around the carbinol carbon(carbon bonded to the hydroxyl group) the hydrogen bonding of fewer molecules results, and therefore, thhe compound can be vaporized at a lower temperature. A dramatic example of this is found in the four carbon alcohols, 1-butanol, 3-Methyl-1-Propanol, 2-Butanol, and 2-Methyl-2-Propanol. Each of these have a boiling point of 118, 108, 100, and 83 respectively.
Polyhydric alcohols have higher boiling points when compared to similar sized monohydric alcohols. This is easily explained since we have more than one Hydroxyl group in the molecule there would be extensively higher hydrogen bonding between the alcohol molecules.


Physical properties of Aldehydes and Ketones
Boiling Points
None of the Hydrogen atoms conected to an aldehyde or ketone are bonded to an Oxygen or Nitrogen so they do not attract other molecules with the strong Hydrogen bonding. For this reason the aldehydes do not have as high a boiling point for the same sized alcohol that does have a Hydrogen bonded to an Oxygen. However aldehydes and ketones do have the carbonyl structure which is polar since the Oxygen is much higher in electronegativity than carbon atom. Therefore aldehydes and ketones will exhibit dipole-dipole interactions as well as the weak London dispersion intermolecular forces which make them have higher boiling points compared to the hydrocarbons and the ethers. For example, propanal and acetone have boiling points of 49oC and 56oC respectively. The difference is due to the slightly higher molecular mass of acetone. This is to be compared with 1-propanol of 97oC and Ethyl Methyl Ether at 8oC and butane at 0oC. The extra strong hydrogen bonding between the 1-Propanol molecules would account for its higher boiling point.

Esters...no Hydrogen bonding -> very volatile, low BP. Polar molecules, therefore short are soluble in water.
Amides...N-H bond is polar with extensive hydrogen bonding -> highly soluble and all molecules have higher BP than alkanes.
Amines...Hydrogen bonding present in Primary and secondary -> soluble (when short) and higher than alkane boiling points. Tertiary amines are very similar to alkanals (but branched -> less dense packing etc).
Halogenoalkanes...short molecules will be soluble due to polar bonds, BP will be somewhat higher


The NH bond is not as polar as the OH bond in alcohols, so amines boil at lower temperatures than alcohols of comparable molecular masses. Amines which have low molecular masses are soluble in water because of hydrogen bonding. This is also why trimethylamine has a lower boiling point than dimethylamine (even though its bigger), becuase there is no NH bond to cause significant hydrogen bonding.

Dimethyl ether was the "ether" once used as an anesthetic. The simple ethers above have very low boiling points because hydrogen bonds do not exist between neighboring molecules. Becuase of this, the boiling point of ethers are much lower than comparable alcohols. For example, 1-butanol (CH3CH2CH2CH2OH) boils at 117 °C, much higher than its isomer, diethyl ether, which boils at 34.5 °C.

• Oxygen is sp3 hybridized
o The C-O-C bond angle is slightly larger than 109.5
Physical Properties
• No Hydrogen bonded to Oxygen
• No H-bonding between molecules
• Lower b.p. than alcohols of same MW.
• Slightly higher b.p. than hydrocarbons
o Dipole-dipole interation
 CH3OCH3 -25 CH3CH2CH3 -45
 CH3CH2OCH2CH3 34 CH3CH2CH2CH2CH2CH3 36

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http://www.chem.wisc.edu/~newtrad/Cu...t/AIDSpp1.html
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