Carbon atoms more substituted

The observed regioselectivity of hydroboration results in part from steric factors — the bulky boron-containing group can approach the less substituted carbon atom more easily. 

Thus, any substitution of the isocyanate molecule which renders this carbon atom more positive should increase the reactivity of the isocyanate. This was verified by Baker (3), who showed that reactivity greatly increased in the following order ... 

The cyclopropenium cation (45, M = C) is a well established aromatic system with a high aromatic stabilization energy of 58.7 kcalmol-1 [equation 18, M = C, at B3LYP/6-311++G(2d,2p)]70. What happens when one carbon atom is substituted by silicon At HF/STO-2G the monosilacyclopropenium cation, 46, has a rather delocalized structure with r(C-C) = 1.393 A, r(C-Si) = 1.722 A and with CSiC and SiCC bond angles of 46.3° and 66.9°, respectively413. However, according to equation 19 which measures the effect of delocalization, 46 is destabilized by 11 kcalmol-1 (HF/3-21G//HF/STO-2G) while 45, M = C is stabilized by 36 kcalmol-1. Unfortunately, this study used a very low computational level and should be repeated using more reliable methods. 

Kerogens isolated from the Fig Tree cherts produced very complex mixtures of pyrolysis products, dominated by a series of methyl branched alkenes with each member of the series having 3 carbon atoms more than the previous member. At each carbon number a highly complex mixture of branched alkanes and alkenes plus various substituted aromatic compounds was found. The highly branched structures may have actually incorporated isoprenoids originally present in the Precambrian microorganisms (Philp Van DeMent, 1983)6>. 

Cycloalkanes that have more than five carbon atoms undergo substitution reactions. In the substitution reactions of alkanes there can be more than one product, but in cycloalkanes, there can only be a single product. 

The alpha-Si y substituted derivatives may be prepared by the Hoffman degradation reaction of the amide containing one carbon atom more than the resulting a/p/ia-allylamine derivative. 

Examples of 1,1-dichlorocyclopropanes (largely selected from the more recent literature) prepared from the alkenes substituted by at least two different substituents, are collected in Table 24. These substituents are joined to vinylic carbons or to carbon atoms more distant from the double bond. These compounds usually form 1,1-dichlorocyclopropane derivatives in good yield, yet some substituted alkenes are known to react in a different, difficult-to-predict manner two examples are described below. 

As wUl be seen later, there are also fuUerenes with one or more carbon atoms being substituted by other elements. In fuUerene nomenclature these molecules are described by prefixing to the name the designating syllable for the respective hetero atom, for ejample, aza[70]fullerene C69N, bis(aza[60]fullerene) (C59N)2, bora[60]fuUerene C59B, or phospha[60]fuUerene C59P. 

In the kinetic studies described in Tables 5.1 and 5.2, the reactions were forced to follow either the SnI or the Sn2 mechanism to yield the relevant rate data. In general, as we said earlier, the two mechanisms compete. This means that the faster mechanism predominates. So, methyl and primary alkyl substrates generally react by the Sn2 mechanism, and tertiary substrates almost always react by the SnI mechanism. Secondary substrates can react via either mechanism depending upon the conditions. Exceptions to this rule include compounds with one or more phenyl groups directly attached to the carbon atom undergoing substitution. 

Polynuclear aromatic hydrocarbons such as naphthalene, anthracene, and phenanthrene undergo electrophilic aromatic substitution reactions in the same manner as benzene. A significant difference is that there are more carbon atoms, more potential sites for substitution, and more resonance structures to consider. In naphthalene, it is important to recognize that there are only two different positions Cl and C2 (see 122). This means that Cl, C4, C5, and C8 are chemically identical and that C2, C3, C6, and C7 are chemically identical. In other words, if substitution occurs at Cl, C4, C5, and C8 as labeled in 122, only one product is formed 1-chloronaphthalene (121), which is the actual product isolated from the chlorination reaction. Chlorination of naphthalene at Cl leads to the five resonance structures shown for arenium ion intermediate 127. 

When weaker, typically neutral, nucleophiles or less reactive carboxylic acid derivatives are involved, the reaction may be catalyzed by acid as outlined in Scheme 20.1. The oxygen atom of the acyl group in 3 is first protonated to give the cation 6 in which the carbon-oxygen bond is even more highly polarized and the acyl carbon atom more electrophilic than in 3. Nucleophilic attack by a neutral nucleophile, H-Nu, then gives the tetrahedral intermediate 7, which will transfer a proton to a base, B, in the reaction mixture to give 8. The details of how the tetrahedral intermediate 8 is converted to the substitution product 5 will depend upon the... 

In other words, the transition state for this step will bear partial carbocationic character. This explains why the water molecule is observed to attack the more substituted carbon. The more substituted carbon is more capable of stabilizing the partial positive charge in the transition state. As a result, the transition state will be lower in energy when the attack takes place at the more substituted carbon atom. The proposed mechanism is therefore consistent with the observed regioselectivity of halohydrin formation. 

One or more hydrogen atoms can be substituted by a carbon atom or chain of hydrocarbons, in which case they are called isoparaffins or isoalkanes. 

Both give TM 31 on treatment with HBr as the cation A reacts preferentially with Br" at the less substituted carbon atom to give the more substituted double bond. Think again. 

Amines also react with epoxides at the less substituted carbon atom. As a slightly more testing problem, suggest a synthesis of the alcohol (TM 165) whose derivatives are used in disinfectants ("phemeiide" etc.). 

Terminal alkyne anions are popular reagents for the acyl anion synthons (RCHjCO"). If this nucleophile is added to aldehydes or ketones, the triple bond remains. This can be con verted to an alkynemercury(II) complex with mercuric salts and is hydrated with water or acids to form ketones (M.M.T. Khan, 1974). The more substituted carbon atom of the al-kynes is converted preferentially into a carbonyl group. Highly substituted a-hydroxyketones are available by this method (J.A. Katzenellenbogen, 1973). Acetylene itself can react with two molecules of an aldehyde or a ketone (V. jager, 1977). Hydration then leads to 1,4-dihydroxy-2-butanones. The 1,4-diols tend to condense to tetrahydrofuran derivatives in the presence of acids. 

Epoxide opening with nucleophiles occurs at the less substituted carbon atom of the oxlrane ting. Cataiytic hydrogenolysis yields the more substituted alcohol. The scheme below contains also an example for trons-dibromination of a C—C double bond followed by dehy-drobromination with strong base for overall conversion into a conjugated diene. The bicycKc tetraene then isomerizes spontaneously to the aromatic l,6-oxido[l0]annulene (E. Vogel, 1964). 

---