Element of unsaturation

Parallel p orbitals in ethylene. The pi bond in ethylene is formed by overlap of the unhybridized p orbitals on the jp -hybrid carbon atoms. This overlap requires the two ends of the molecule to be coplanar. 

Distinct isomers resulting from C—C double bonds. The two isomers of 2-butene cannot interconvert by rotation about the carhon-carhon double bond without breaking the pi bond. 

Alkenes are said to be unsaturated because they are capable of adding hydrogen in the presence of a catalyst. The product, an alkane, is called saturated because it cannot react with any more hydrogen. The presence of a pi bond of an alkene (or an alkyne) or the ring of a cyclic compound decreases the number of hydrogen atoms in a molecular formula. These structural features are called elements of unsaturation. Each element of unsaturation corresponds to two fewer hydrogen atoms than in the saturated formula. 

Consider, for example, the formula C4H8. A saturated alkane would have a C H(2 +2) formula, or C4H a. The formula C4H8 is missing two hydrogen atoms, so it has one 

When you need a structure for a particular molecular formula, it helps to find the number of elements of unsaturation. Calculate the number of hydrogen atoms from the saturated formula, C H(2i,+2). sc how many are missing. The number of elements of unsaturation is simply half the number of missing hydrogens. This simple calculation allows you to consider possible structures quickly, without always having to check for the correct molecular formula. 

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Other terms that mean the same thing as the index of hydrogen deficiency include elements of unsaturation sites of unsaturation and the sum of double bonds and rings... 

In general, radical cations of alkenes or cyclopropanes produce nonconjugated radicals, while those of dienes give rise to allyl radicals, and those of vinylcyclopropane or vinylcyclobutane systems generate either allylic or nonconjugated radicals with an additional element of unsaturation. In contrast to the most thoroughly characterized nucleophilic substitution of appropriate neutral molecules. 

Alkynes are hydrocarbons that contain a carbon-carbon triple bond. A triple bond consists of a cr bond and two tt bonds. The general formula for the alkynes is C li2n-2- The triple bond possesses two elements of unsaturation. Alkynes are commonly named as substituted acetylenes. Compounds with triple bonds at the end of a molecule are called terminal alkynes. Terminal —CH groups are called acetylenic hydrogens. If the triple bond has two alkyl groups on both sides, it is called an internal alkyne. 

Examination of the molecular formula C14H2602 reveals that the compound has an index of hydrogen deficiency of 2. Because we are told that the compound is a carboxylic acid, one of these elements of unsaturation must be a carbon-oxygen double bond. The other must be a carbon-carbon double bond because the compound undergoes cleavage on ozonolysis. Examining the products of ozonol-ysis serves to locate the position of the double bond. 

Compound A (C4H602) has an index of hydrogen deficiency of 2. With two oxygen atoms and a peak in the infrared at 1760 cm-1, it is likely that one of the elements of unsaturation is the carbon-oxygen double bond of an ester. The H NMR spectrum contains a three-proton singlet at 8 2.1 ppm, which is consistent with a CH3C unit. It is likely that compound A is an acetate ester. 

The Orbital Description of the Alkene Double Bond 286 7-3 Elements of Unsaturation 287 7-4 Nomenclature of Alkenes 289 7-5 Nomenclature of Cis-Trans Isomers 291 Summary Rules for Naming Alkenes 293 7-6 Commercial Importance of Alkenes 294 7-7 Stability of Alkenes 296 7-8 Physical Properties of Alkenes 302... 

Determine the number of elements of unsaturation in the molecular formula C4Hg. Give all nine possible structures having this formula. Remember that... 

Heteroatoms (hetero, different ) are any atoms other than carbon and hydrogen. The rule for calculating elements of unsaturation in hydrocarbons can be extended to include heteroatoms. Let s consider how the addition of a heteroatom affects the number of hydrogen atoms in the formula. 

Nitrogen A nitrogen atom can take the place of a carbon atom in the chain, but nitrogen is trivalent, having only one additional hydrogen atom, compared with two hydrogens for each additional carbon atom. In computing the elements of unsaturation,... 

The formula C4H9N is like a formula with 4 carbon atoms, with saturated formula C45H9+2. The formula C4H9N has one element of unsaturation, because it is two hydrogen atoms short of the saturated formula. 

For each of the following molecular formulas, determine the number of elements of unsaturation, and draw three examples. 

Draw and name alkenes, and calculate the elements of unsaturation. 

A structural feature that results in two fewer hydrogen atoms in the molecular formula. A double bond or a ring is one element of unsaturation a triple bond is two elements of unsaturation, (p. 287)... 

A triple bond gives an alkyne four fewer hydrogens than the corresponding alkane. Its molecular formula is like that of a molecule with two double bonds CnH2n-2-Therefore, the triple bond contributes two elements of unsaturation (eu) (Section 7-3). 

If the molecular formula is known, use it to determine the number of elements of unsaturation (see Section 7-3). The elements of unsaturation suggest rings, double bonds, or triple bonds. Matching the integrated peak areas with the number of protons in the formula gives the numbers of protons represented by the individual peaks. 

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