Questions about NMR/Organic Chem

[zsteve]

Could someone help me with the following questions? The textbook is not forthcoming, and my teacher's a bit vague:
- Why can't we react asymmetric alkenes with HCl to produce chloroalkanes? Why must it always be done by hydration reaction and then substitution with Cl2 to produce mixtures of different chlorinated compounds? I believe it is because the former method produces only tiny amounts of the desired compound (where Cl is bonded to the end of the chain) but confirmation anyone?
- Why is chemical shift positive in NMR? If a downfield shift corresponds to less energy required, what about peaks to the right of TMS? (more energy required) Is it possible?
- Does 1H NMR work because other nuclei do not resonate at the wavelengths examined by 1H NMR? (Hence we only get signals from 1H at the radio wavelengths used?)

Any help here would be very much appreciated

[mahler004]

- Look up Markovnikov's rule. The chlorine ends up on the most substituted carbon (or at least that's the major product.) There are ways you can get around this, but they are a bit beyond VCE.

- Chemical shift is positive in NMR by convention, really. Keep in mind that the ppm measurement is really just a way of normalising measurements on different spectrometers. TMS is set as '0' because the hydrogens are very well shielded, and not many protons will have a smaller chemical shift then TMS.

You can indeed have protons that are shifted to the right of TMS (you'll see this a lot in the spectra of proteins - the methyl protons are typically at around -1 ppm.)

- Yep, basically.