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Biochemistry questions .

  1. Oct 1, 2016 #1
    Hello .
    1) -
    i am having some problems with biochemistry (first month of first semester of first year of medical school) .
    the course contains a large number of spontaneous chemical reactions especially on -OH , -COOH , -COH
    ... groups on glucides , Amino acides and so on .... when i ask the professors they say that they don't have time to explain the foundations of those reactions and that i just need to memorise them the way they are .
    now they said that for me to understand this i need to look for :
    - notion of electronegativity
    - atomic models and electron distribution
    - nucleophile and electrophile components and why they are like that .
    and so on ...
    now i looked for this for sometime on the Net and i just don't seem to be able to find what i need .
    can someone please guide me to some good documents to explain these things ?

    2) - a chiral molecule is a molecule that is not superposable with it's image in a mirror since it has no axe of symmetry , now the course then links this to Asymmetrical carbon atoms which is logical since a C that is linked to 4 different component always gives a Chiral molecule . now what if you would take a molecule like this one
    Code (Text):


    now that Carbon atom is not Asymmetrical , so the molecule is Achiral and it's not a chiral molecule , yet it fits the definition , that molecule is not superposable with it's image in a mirror .
    and if we take the D-glucose for example : (fisher representation)
    Code (Text):

                                       H - C = O
                                       H - *C - OH
                                    OH - *C - H
                                       H - *C - OH
                                       H - *C - OH
                                         CH2OH                           (*C : asymmetrical carbon atom)

    if we change the configuration of a single *C the proprieties of the molecule change , yet what if we rotate the upper link , ( - COH : grouple aldehyde) that should not change the molecule and it's as if we didn't do a thing since that carbon atom n#1 is not asymetrical , even the configuration of the bottom CH2OH doesn't matter in stereo-isomerism . i don't understand this .

    i also have a problem with mutarotation and Anomers and the behavior of glucides in water , i think i would understand that if i understand the notions in 1)- .

    i am used to Math and physics and those were well founded for me so my mind accepted them , now though the quantity of information is big yet i don't understand the foundations , i can memorise it all as it is and just pass the exam but i know that's not good .

    thank you very much for your attention .
  2. jcsd
  3. Oct 1, 2016 #2


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    Staff: Mentor

    Have a look at https://www2.chemistry.msu.edu/faculty/reusch/virttxtjml/intro1.htm
    or pick up a good textbook on organic chemistry.

    Yes it is: you are allowed to rotate the molecule. Don't just look at the mirror image, but think if you build that mirror image molecule, would it be the same as the original or not.

    I don't understand the question here.

    I suggest you buy a molecular model kit and build these molecules. It will greatly help in understanding what is going on and what is different from one molecule to the next. And honestly, playing with molecular models is fun :wink:
  4. Oct 1, 2016 #3


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    Science Advisor
    Gold Member

    I really don't mean to sound snarky, but I have to ask: were you not required to take organic chemistry to get into medical school? Most of the questions you ask are at the level of a first-semester organic chemistry course.
    1) This question is way too general and encompasses material from introductory general chemistry as well as the first few months of first semester organic chemistry. You might consider getting some remedial tutoring or taking classes at a community college.
    2) The molecule in question is ethanol. I assure you that it is achiral and it's superposable onto its mirror image. You might want to get a cheap molecular model kit and build some of these molecules so that you can hold them in your hand. Another option is to download a molecule building program like Avogadro, where you can build molecules and manipulate them on your computer.
    Second part of 2) The molecule in question is D-glucose. Again, I encourage you to get either a physical molecular model kit or a program where you can actually view these molecules from different angles. If you have specific questions after that, you can ask them here and you'll probably get more informative answer. As it stands right now, the questions you have are probably too broad for an online forum like this.

    One other thing: the English word for glucide is carbohydrate. That might help you if you're searching English-language websites for information.
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