Integrated SequencePhysics Chemistry Organic Biology

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Special points of emphasis

Water

Organic Acids and Bases

Reactions of Alcohols and Ethers

Like water, alcohols are amphoteric, able to serve as either a proton donor or a proton receiver. The basicity of hydroxyl group, its ability to receive a proton, permits the conversion of the hydroxyl group into a leaving group. A protonated alcohol (the conjugate acid of the alcohol) is called an alkyloxionium ion, formed in the presence of a strong acid. Formation of alkyloxonium can be is followed by the departure water as a leaving group forming a carbocation. Dehydrohalogenation of an alcohol is an SN1 substitution mechanism where water is the leaving group and halide ion serves as the nucleophile.



Work, Energy, and Power

Electricity

The Chemical Bond

Thermochemistry

Chemical Thermodynamics and the Equilibrium State

Chemical Kinetics

Reactions of Alkyl Halides

Reactions of Alcohols and Ethers

The dehydration of an alcohol is an E1 elimination reaction. Therefore, acid-catalyzed dehydration of alcohols involves a carbocation intermediate, and rearrangement to form more stable carbocation can occur. Acid catalyzed dehydration of alcohols is both regioselective (Zaitsev's Rule favoring the highly substituted alkene) and stereoselective (favoring the trans product). The basis for both the regioselectivity and stereoselectivity is that the most probable state involves minimizing internal energy (minimum enthalpy, minimum free energy).

Highly substituted alkenes are more stable because carbon-carbon bonds in sp3 hybridized alkyl groups release electron density inductively to sp2 hybridized carbons which are more electron withdrawing than alkyl groups. The greater the number of alkyl substituents of the double bond, the more stabilization can occur. (The movement of electron density to satisfy an electron withdrawing substituent corresponds to a decrease in electric potential energy. In terms of thermochemistry, this means lowered internal energy, lowered enthalpy, and, in terms of thermodynamics, lower free energy (i.e. favored by equilibrium)).

Similar reasoning explains the stereoselective preference with elimination reactions to form the trans isomer in greater yield than the cis. Here the decisive electrostatic interaction at the molecular level involves van der Waals repulsion between alkyl groups on the same side of the double bond. Thermodynamics favors the lower energy trans form which minimizes these repulsions.




Reactions of Alcohols and Ethers

Bioenergetics and Cellular Respiration

Integration of Metabolism

Read for comprehension (not memorization). Dehydration of alcohols is a very significant reaction in biochemistry. For example, after the malonyl group has combined with an acetyl group in fatty acid biosynthesis, a ketone carbonyl is reduced to a hydroxyl group. This hydroxyl group then serves as the site for dehydrogenation. (The vinylic group is then hydrogenated to form an aliphatic chain). Another example of dehydrogenation of alcohols occurs in the citric acid cycle, where the first step in the isomerization of citrate to isocitrate is the dehydration of an alcohol.







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