[Araconan]

So I've recently just began studying Organic Chemistry, and one of the first things that I'm completely confused on is the concept of electrophiles and nucelophiles.

Take for example CH₃Cl. My textbook says that it's an electrophile, but can't you make the argument that because chlorine has 3 lone pairs of electrons (and because it'll be negatively polarized), it's actually a nucleophile?

But then for the compound CH₃CH=O, the textbook said that it was both an electrophile and a nucleophile. Following the logic of the first example, since the electronegative chlorine was ignored, shouldn't the electronegative oxygen also be ignored, and the compound be only a electrophile? Why is in this case, the electronegative oxygen also makes it an nucelophile, but the electronegative chlorine didn't make chloromethane an nucleophile?

Finally, HBr is said to be an electrophile, because of the positive charge of hydrogen. But isn't there also a negative charge on bromine, so shouldn't it be both an electrophile, and a nucleophile?

I guess my confusion really lies in that shouldn't a compound always be both an electrophile, and a nucelophile, since if a polarized positive regions exists, there must also be a polarized negative region?

[discodermolide]

Take CH₃Cl, the C-Cl bond is polarised towards the Cl atom making the C atom slightly positive thus susceptible to attack by a nucleophile. Therefore the compound is an electrophile.
For acetaldehyde you have the electrophillic site at the aldehyde carbon which is attacked by nucleophiles. Also the CH₃ protons of acetaldehyde are "acidic" and can be removed by a base thus forming an enolate, The enolate is a nucleophile at the carbon alpha to the C=O group.
With HBr it depends on the reaction you are doing. It is a very strong acid and I don't think the concept of electrophiles and nucleophiles really applies here. OK HBr will protonate an alcohol OH making the carbon in C-OH electrophillic and it will be attacked by Br^-, in this case a nucleophile, but the mechanism is slightly different from the other examples.

[Araconan]

For CH₃Cl though, wouldn't the chlorine also become slightly negative, and therefore able to bond to an electrophile, and so the compound is also a nucleophile?

So doesn't that go back to the point where shouldn't all compounds be both electrophiles and nucleophiles, since you'll always have a slightly positive (making compound electrophile) region (like C in CH₃Cl) and a slightly negative (making compound nucelophile) region (like Cl in CH₃Cl)?

Is there some kind of rule that I'm missing here that negates one of the possibilities?

[discodermolide]

The chlorine in methyl chloride is not nucleophillic. Consider it's reaction with a nucleophile like sodium azide. You will get methyl azide and sodium chloride. So the chloride is removed as a salt and cannot act as a nucleophile.
Methyl chloride will not attack methylazide with the chlorine and displace the azide.