MadSci Network: Chemistry
Query:

Re: What causes polarity in an alcohol?

Date: Fri Feb 11 00:19:48 2005
Posted By: Neil Saunders, Research fellow
Area of science: Chemistry
ID: 1103167100.Ch
Message:

Hi Anne,

Let's take a look at alcohols. First, what is an alcohol? It's defined as a compound where a hydroxyl (-OH) group is attached to a saturated carbon atom. We can represent an alcohol like this:

     R
     |
   R-C-O-H
     |
     R

Those 'R' groups could be all kinds of things. If they were all hydrogen, we would have the simplest alcohol, methanol, CH3OH. Or we could have 2 hydrogens and a methyl (CH3) group to give ethanol, CH3CH2OH.

You probably know that the bonds we have drawn are called covalent bonds. This means for example, that in the C-O bond, electrons around the C and O atoms have paired with each other. The way we picture this is that the bond is a "cloud" surrounding the C and the O, in which electrons from either atom can be found. Now, it so happens that the electrons in this bond spend more of their time close to the O atom than to the C atom. This is because of a property called electronegativity - O is more electronegative than C, so electrons are drawn towards the O atom. What is the effect of this? Well, electrons have a negative charge, so the O end of the bond has a small negative charge, whilst the C end has a small positive charge, because electrons are drawn away from it. We call this a dipole moment and it explains the polarity in alcohols.

The same thing happens between the O and the H atom in the O-H bond. In fact, this also happens in water and this website has a nice picture of a water molecule which shows what I mean:

http://bio.winona.msus.edu/bates/Bio241/images/figure-02-08.jpg

OK - so why is polarity important for biologists? Well, the small positive and negative charges in polar bonds allow molecules that contain them to attract each other and stick together. This is called a hydrogen bond and it explains the properties of water and why molecules like ethanol can mix with water, whilst molecules like ethane cannot. This webpage has a diagram showing ethanol hydrogen bonding to water:

http://www.elmhurst.edu/~chm/vchembook/162othermolecules.html

Hydrogen bonds are common in biology - for instance, the bonds that bind 2 strands of DNA together are hydrogen bonds. Hydrogen bonds are also found in proteins and enzymes - they help the enzyme to keep its 3D shape and catalyze reactions. And all these hydrogen bonds come about as a result of polarity.

I hope this has helped you - if you have more chemistry problems, a nice website is General Chemistry Online.
Neil


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