[Lauren_Summer]

Hi, I have some chemistry questions and if anyone can answer any of them i will love you forever!

1) Thiopental passes more rapidly through the blood brain barrier than pentobarbital, this is indicated by:
a. the low heptane/water partition coefficient of pentobarbital in comparison to thiopental
b.the large fraction of pentobarbital that remains non-ionised at pH 7.4
c.the low relative molecular mass of thiopental
d.the pKa of pentobarbital is 8.1, that of thiopental is 7.6
e. none of these physiochemical properties explain this observation

2) Which of the following statements is true regarding glucose?
a. it can be reduced to gluconic acid by glucose reductase
b. it has a strong chromaphore
c. it is usually isolated by reverse phase partition HPLC
d. it is usually analysed by indirect spectroscopic methods
e. it is a hydrocarbon

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[-Kav-]

(Original post by Lauren_Summer)
Hi, I have some chemistry questions and if anyone can answer any of them i will love you forever!

1) Thiopental passes more rapidly through the blood brain barrier than pentobarbital, this is indicated by:
a. the low heptane/water partition coefficient of pentobarbital in comparison to thiopental
b.the large fraction of pentobarbital that remains non-ionised at pH 7.4
c.the low relative molecular mass of thiopental
d.the pKa of pentobarbital is 8.1, that of thiopental is 7.6
e. none of these physiochemical properties explain this observation

2) Which of the following statements is true regarding glucose?
a. it can be reduced to gluconic acid by glucose reductase
b. it has a strong chromaphore
c. it is usually isolated by reverse phase partition HPLC
d. it is usually analysed by indirect spectroscopic methods
e. it is a hydrocarbon

Man, this is a real minefield, but here goes:

For Q1, thiopental has a higher RMM than pentobarbital, and smaller molecules tend to pass through the BBB faster, so (c) doesn't explain the result. (b) and (d) are essentially the same point, both suggesting that, at any given time, pentobarbital is more likely to exist in its non-ionised form than thiopental in in vivo. Since the non-ionised form is more hydrophobic in each case than the ionised form, and hydrophobicity facilitates the crossing of the BBB, neither of these explain the result. A lower heptane/water coefficient for pentobarbital than for thiopental indicates that pentobarbital is the more hydrophilic molecule, and thus the molecule that has the greater difficulty crossing the BBB. As such, (a) explains the result. Really worth checking through my logic for gaping holes, there, but I think it's OK.

For Q2, (a) can be easily ruled out as glucose to gluconic acid is an oxidative process, (b) can be easily ruled out as glucose is colourless, and (e) can be easily ruled out as glucose possesses several oxygen atoms. Between (c) and (d) is a real judgement call, though, since they're so vague. My best guess is that glucose is usually analysed by chromatographic methods. Such methods are cheap, straightforward and very reliable for monosasccharides. Cocking about with NMR machines or the like would seem like a massive waste of time, expertise and money. As such, (d) is likely to be false. That leaves (c), separation by reverse phase HPLC. This stands pretty well to reason, given the large scale on which glucose is likely to be produced, and given the environmental concerns surrounding large-scale normal phase chromatography.

I hope these answers find you in time.

[Lauren_Summer]

(Original post by -Kav-)
I hope these answers find you in time.

Oh my god that was so nice of you!!

[LearningMath]

Q2)d) As far as I know (I'm only an undergrad and so the experience I'm basing this off is just a handful of papers and teaching labs! :P) glucose concentration is usually determined via a glucose electrode (in various forms!) or often via indirect spectroscopic method (usually oxidation by glucose oxidase, with the product H2O2 then used to oxidase a chromogenic substrate eg TMB in a reaction catalysed by horseradish peroxidase... the resulting indirect absorbance change is then related to the H2O2 and :. glucose conc by the beer-lambert law). I mean its going to depend on the lab and what you're trying to do but I have the impression that indirect spectroscpic methods are not uncommon for analysing metabolite concentrations and so d) seems reasonable?

I appreciate this thread is old and you probably no longer care! :P