Redox Reactions and Enzymes – Alcohols

by Adam Le Gresley, PhD

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    00:01 Right. Let’s look briefly at RedOx reactions and enzymes. So, we’re going to touch upon biochemical reactions in living organisms which are essentially transfers of energy from one thing to the other. Often, they occur together linked in the chain in what is referred to as a RedOx or oxidation-reduction reaction.

    00:21 As we said before when we were looking at other types of ionic reaction, one chemical is oxidised and its electrons are then passed to another chemical and is thus reduced. Such coupled reactions are referred to as RedOx reactions and are very important to every facet of every element of even the most basic biochemical process such as the ability for us to convert sugar into adenosine triphosphates.

    00:50 Important metabolic processes, such as glycolysis, which involves the conversion of a sugar into acetyl co-enzyme A, the Krebs cycle which involves the conversion amongst other things of acetyl co-enzyme A into NADH and finally, electron-transport phosphorylation which uses NADH in the conversion of ADP to ATP. This all involves the transfer of electrons via RedOx reactions and is very important.

    01:23 These are often catalysed, as we’ll see, and this goes back to the original discussion about protein and, of course, DNA structure from whence proteins are derived.

    01:33 Enzymes are proteins that can accelerate a chemical reaction and effectively are just organic or biological catalysts. Let’s have a quick look at oxidation and reduction again in terms of our alcohols and in terms of our carboxylic acids.

    01:50 If we look at the lower set, you’ll see we have, at one end, a terminal, primary alcohol.

    01:57 Next along is an aldehyde and finally, is a carboxylic acid.

    02:02 At the top, we have examples of alkanes, alkenes and alkynes. And in much the same way that we consider an alkyne to be partially more oxidised than an alkane or an alkene, we also consider the carboxylic acid to be more oxidised than an aldehyde or primary alcohol.

    02:24 Something we also have is H+ ions along, so reduction also becomes the gain of H and the oxidisation the loss of H. So, let’s have a quick look at a biological application of this.

    02:37 You recall me referring earlier to what happens within the body. So, this is a very simple biological process by which alcohol, which is highly fat soluble amongst other things and, of course, can disrupt ion channels creating, again, a degree of toxicity, can be oxidised via alcohol dehydrogenase shown here. This converts the alcohol into an aldehyde, in this case, acetaldehyde to use its old name or ethanal to use the accepted IUPAC name.

    03:09 This aldehyde, which is one of the things that’s actually responsible for the hangover associated with excess consumption of alcohol, is then converted by aldehyde dehydrogenase ALDH2 into the correlating carboxylic acid, which in of itself, can either serve as a fuel for the Krebs cycle or itself can be excreted in the urine.

    03:34 Another application of alcohol in chemistry in this particular regard is the use or the old use of oxidising agents within a breathalyser. These days it’s a little bit more advanced, but previously, you used to have a bag containing potassium dichromate crystals here which are yellowy-orange in colour. Of course, the reaction with an alcohol and its conversion into a carboxylic acid is spontaneous on contact with these and the result is that the oxidised, remember chromium and its +6 oxidation state, can be reduced by the alcohol in the process of its own oxidation to bring the oxidation state of chromium down to 3+. This, for reasons we will not go into during the course of this course, convert the colour of the chromium salt from yellowy-orange down to green. Chromium (III) ions, that is to say chromium in their 3+ oxidation state, are green and chromium in its 6+ oxidation state is a yellowy-orange colour.

    04:38 And, for the purposes of reference, it allows you to, therefore, calculate the concentration of alcohol assuming that you have an approximate correlation between the amount of alcohol in your breath and the amount of alcohol you can infer within your bloodstream.

    04:57 Newer breathalysers use spectroscopic analysis of the breath trapped in the sample and this is an analytical technique which we will not discuss here.

    05:06 Approximately, 2000mL of breath contains the same amount of alcohol as approximately 1mL of blood. However, it should be borne in mind that we are not all the same and it should be considered that, for example, somebody who has a particularly large volume of blood, more so than others, will perhaps give a result which could be considered inaccurate.

    About the Lecture

    The lecture Redox Reactions and Enzymes – Alcohols by Adam Le Gresley, PhD is from the course Organic Chemistry.

    Included Quiz Questions

    1. The electron transport chain (ETC) plays an important role in the synthesis of alcohols and haloalkanes in the living systems.
    2. Redox reactions involve electron transfer from one molecule to another during cellular respiration in the living systems.
    3. In a redox reaction involving a pair of chemicals, one molecule gets reduced while other is oxidized.
    4. In the biological systems, the redox reactions take part in the complete breakdown of glucose to generate cell energy in the form of ATP (Adenosine triphosphate).
    5. The respiratory chain in the living systems is a perfect example of redox reaction with sequential oxidation-reduction steps coupling the electron transfer for energy generation.
    1. Alcohol dehydrogenase and aldehyde dehydrogenase enzymes
    2. Alcohol dehydrogenase enzyme
    3. Aldehyde dehydrogenase enzyme
    4. Krebs cycle
    5. A strong oxidizing agent like K2Cr2O7
    1. In the presence of K2Cr2O7 crystal, ethanol gets oxidized to acetic acid, accompanied by a color change from yellow [Cr(VI)] to green [Cr(III)]
    2. In the presence of alcohol dehydrogenase
    3. In the presence of aldehyde dehydrogenase
    4. In the presence of both alcohol dehydrogenase and aldehyde dehydrogenase
    5. In the presence of aldehyde

    Author of lecture Redox Reactions and Enzymes – Alcohols

     Adam Le Gresley, PhD

    Adam Le Gresley, PhD

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