Alcohol constitutional isomers

Except for the biochemical exanple just cited, the stnactures of all of the alcohols in Section 5.9 (including those in Problem 5.13) were such that each one could give only a single alkene by p elimination. What about elimination in alcohols such as 2-methyl-2-butanol, in which dehydration can occur in two different directions to give alkenes that are constitutional isomers Here, a double bond can be generated between C-1 and C-2 or between C-2 and C-3. Both processes occur but not nearly to the sane extent. Under the usual reaction conditions 2-methyl-2-butene is the major product, and 2-methyl-1-butene the minor one. 

Interestingly, the product actually isolated from alkyne hydration is not the vinylic alcohol, or enol (ene + ol), but is instead a ketone. Although the enol is an intermediate in the reaction, it immediately rearranges to a ketone by a process called keto-enol tautomerisni. The individual keto and enol forms are said to be tautomers, a word used to describe constitutional isomers that interconvert rapidly. With few exceptions, the keto-enol tautomeric equilibrium lies on the side of the ketone enols are almost never isolated. We ll look more closely... 

Isopropyl alcohol is a constitutional isomer of propyl alcohol. 

Constitutional (formerly structural) isomerism is encountered when polymers have the same overall chemical composition (i.e., same molecular formula) but differ in connectivity— the order in which the atoms are connected to each other. Polyacetaldehyde, poly(ethylene oxide), and poly(vinyl alcohol) are constitutional isomers. The first two polymers are... 

SAMPLE SOLUTION (a) Begin by considering the ways in which two carbons and one oxygen may be bonded. There are two possibilities C—C—O and C—O—C. Add the six hydrogens so that each carbon has four bonds and each oxygen two. There are two constitutional isomers ethyl alcohol and dimethyl ether. 

For three parts, there are two constitutional isomers that will give the desired alcohol. For two parts, only one alkene will give the desired alcohol. 

B-9. Which of the following compounds will undergo hydrolysis (SN1) to give a mixture of two alcohols that are constitutional isomers ... 

The hydroboration of alkenes, in which the Ca=C is not symmetrically substituted can lead to constitutionally isomeric trialkylhoranes. This is because the new C—B bond can form either at the Ca or at the (A of the Ca=C double bond. In the oxidation/hydrolysis sequence that follows, constitutionally isomeric alcohols are produced. In one of them, the OH group binds to Ca and in the other it binds to C. If only one constitutional isomer of the trialkylbo-rane and consequently only one constitutional isomer of the alcohol is to he produced, the hydroboration step must take place regioselectively. Whether regioselectivity occurs is determined by steric and electronic effects. 

There are nine constitutional isomers with the empirical formula C3H60. However, since two of the compounds exist as pairs of stereoisomers, the total number of isomers equals eleven. The two methyloxiranes, A and B, are enantiomers, they are stable and very volatile. Acetone (C) is in equilibrium with its enol tautomer (propen-2-ol, D). Similarly, the two diastereomeric enols ( )- and (Z)-prop-l-en-l-ol (E and F) are in equilibrium with their tautomer propanal (or propionaldehyde, G). The other isomers are (last group of compounds from left to right) methoxyethene, prop-2-en-l-ol (commonly known as allyl alcohol), oxetane and cyclopropanol. 

Which of these two structures is correct Both of them satisfy the octet rule and neither has formal charges, so both are predicted to be of comparable stability. On the basis of what we have discussed so far, we cannot predict which is more stable. In fact, both of these compounds are quite stable and can be put in a bottle. But they are different compounds. Ethyl alcohol is the alcohol found in beverages. It is a liquid at room temperature. In contrast, dimethyl ether is a gas at room temperature and is quite poisonous. As was mentioned in Section 1.7, compounds such as these, with the same molecular formula but different arrangements of bonded atoms (different structures or different connectivities), are called constitutional isomers (or structural isomers). Constitutional isomerism is very common in organic compounds. This is another reason why it is necessary to show the structure of the compound under discussion rather than just the molecular formula. 

For each molecular formula, draw all the possible constitutional isomers of alcohols with that formula. Give the IUPAC name for each alcohol. 

Constitutional isomers like butane and isobutane belong to the same family of compounds they are both alkanes. In contrast, constitutional isomers like CH3CH2OH and CH3OCH3 have different functional groups and belong to different families CH3CH2OH is an alcohol and CH3OCH3 is an ether. 

Problem 9.1 Draw all constitutional isomers having molecular formula C4H10O. Classify each compound as a 1 °, 2°, or 3° alcohol, or a symmetrical or unsymmetrical ether. 

When an alcohol has two or three different P carbons, dehydration is regioselective and follows the Zaitsev rule. The more substituted alkene is the major product when a mixture of constitutional isomers is possible. For example, elimination of H and OH from 2-methyl-2-butanol yields two constitutional isomers the trisubstituted alkene A as major product and the disubstituted alkene B as minor product. 

Constitutional isomerism is not limited to alkanes—it occurs widely throughout organic chemistiy. Constitutional isomers may have different carbon skeletons (as in isobutane and butane), different functional groups (as in ethyl alcohol and dimethyl ether), or different locations of a functional group along the chain (.as in isopropylamine and propylamine). Regardless of the reason for the isomerism, constitutional isomers are always different compounds with different properties, but with the same formula. 

There are four constitutional isomers of the saturated acyclic four-carbon alcohols with one Acyclic compounds contain no rings. —OH per molecule. Write the structural formula of each, and identify each as primary, secondary, or tertiary. Name each isomer. 

Only constitutional isomers are requested, not stereoisomers, and only structures with an alcohol group... 

Problem 13.5. How many alcohols exist for the molecular formula C4Hj(,0 Show only one structural formula for each constitutional isomer. 

Ans. This problem is not the same as Problem 13.23. The latter specified ethers with the formula C4H10O but this problem does not have that limitation. Alcohols and ethers are constitutional isomers of each other. The answer to this problem is the sum of the answers to Problems 13.5 (C4H jqO alcohols) and 13.23 (C4H,oO ethers). 

Ans. Compounds I and III are constitutional isomers of C2H4O2 but not in the same family. Compound I is a carboxylic acid and compound III is an aldehyde-alcohol. 

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