Primary, Secondary, and Tertiary Carbons

A carbon which is attached directly to only one other carbon is called a primary (1 ) carbon. If it is attached directly to two other carbons, it is a secondary (2 ) carbon. A carbon is called a tertiary (3 ) carbon, if it is directly attached to three other carbons. 

Though alkanes are not so reactive, they can undergo some reactions by forming intermediates. These intermediates can be alkyl radicals, carbocations, or carbanions. Alkyl radicals are intermediates of free radical reactions. Carbocations (carbonium ions) are species with a positive charge on one of the carbon atoms. A carbanion has a negative charge on one of its carbon atoms. Some major trends are given below  

Alkanes can have different conformations. By analyzing the structure of ethane, we can define certain aspects regarding its conformations. Conformations are different arrangements of the atoms in a molecule, as a result of rotation around a single bond. 

In staggered conformation, the torsional angle is 60 . In eclipsed conformation, each carbon-hydrogen bond is aligned with the carbon-hydrogen bond of the next carbon. 

According to Baeyer strain theory, the stability of a cycloalkane is based on how close its angles are to 109.5°. The closer the angle is to 109.5°, the more stable the cycloalkane. Deviation from this angle can cause angle strain. An increase in the angle strain means a decrease in the stability of the molecule. 

---

Only limited success was achieved in determining the relative reactivity of primary, secondary, and tertiary carbon-hydrogen bonds to sulphonyl nitrenes 8>. Insertion of p-toluenesulphonyl nitrene into 2-methylbutane gave a mixture of products which could not be completely resolved. The ratio of (primary) (secondary + tertiary) = [38 + 39 40 + 41] was 1.53, compared to a ratio of 5.6 for carbethoxynitrene58>, indicating the lowered selectivity of the sulphonyl nitrene relative to the carbethoxynitrene, as might be expected from the possible resonance stabilization of the latter species. 

On a purely statistical basis, we may expect the ratio of products from 3 to correlate with the number of available hydrogens at the various positions of substitution. That is, 4, 5, 6, and 7 would be formed in the ratio 6 3 2 l (50% 25% 17% 8%). However, as can be seen from Table 4-6, the strengths of hydrogen bonds to primary, secondary, and tertiary carbons are not the same and, from the argument given in Section 4-4E we would expect the weaker C-H bonds to be preferentially attacked by CI-. The proportion of 7 formed is about three times that expected on a statistical basis which is in accord with our expectation that the tertiary C-H bond of 2-methylbutane should be the weakest of the C-H bonds. (See Table 4-6.)... 

FIGURE 5.1 RSEs for primary, secondary, and tertiary carbon-centered radicals at 298.15 K, calculated according to Equation 5.3 (all in kJ/mol). 

It should be mentioned that a solvent change affects not only the reaction rate, but also the reaction mechanism (see Section 5.5.7). The reaction mechanism for some haloalkanes changes from SnI to Sn2 when the solvent is changed from aqueous ethanol to acetone. On the other hand, reactions of halomethanes, which proceed in aqueous ethanol by an Sn2 mechanism, can become Sn 1 in more strongly ionizing solvents such as formic acid. For a comparison of solvent effects on nucleophilic substitution reactions at primary, secondary, and tertiary carbon atoms, see references [72, 784]. 

The (n, n ) states of alkanones can provide a test for the application of TET to Type II reactions with no adjustable parameters. The reaction energies of H abstraction from primary, secondary, and tertiary carbons, by these states can be safely calculated as 20.7, 40.3, and 49.7kJmor , respectively. No CT is expected. The rate constants for these abstractions [117] can be reproduced, within a factor of 2.5, with d = 38.0 pm, very close to the theoretical value, 37.3 pm. 

In acid catalyzed reactions reactant shape selectivity reverses the usual order of carbocation reaction rates. Acid catalyzed reactivities of primary, secondary, and tertiary carbons differ. Tertiary carbon atoms form the most stable carbocations therefore, they react much faster than secondary carbon atoms. Primary carbon atoms do not form carbocations under ordinary conditions and therefore do not react. Only secondary carbocations can form on normal paraffins whereas tertiary carbocations form on singly branched isoparaffins. Therefore, in most cases, isoparaffins crack and isomerize much faster than normal paraffins. This order is reversed in most shape selective acid catalysis, that is, normal paraffins react faster than branched ones, which sometimes do not react at all. This is the essence of many applications of reactant or product type shape selective acid catalysis. 

The carbon is electrically neutral, but it does have a greater electronegativity than hydrogen, and so it attracts slightly the electrons within the carbon/ hydrogen bonds. Now suggest the relative order of stability of the primary, secondary and tertiary carbon radicals. tertiary>secondary>primary... 

In the case of methane, there was only one type of hydrogen that the chlorine radical could attack, but in a larger alkane there is often a choice. One of the principal factors that determines which hydrogen will be abstracted is the stability of the resultant carbon radical. Suggest what will be the order of stability of primary, secondary and tertiary carbon radicals. 

In spite of the absence of many examples, the ascending sequence for primary, secondary and tertiary carbon-halogen bond lengths is apparent thus, for C—Cl bonds, CHaCHjCK/i = 1.788 0.002 A), >... 

This order of primary, secondary and tertiary carbon atom effects is an interesting reversal of what is usually observed. In considering the heats of hydrogenation of unsaturated hydrocarbons it was found that the substitution of carbon for hydrogen attached to an unsaturated carbon atom acted so as to increase, apparently, the stability of the compound containing the double bond. An exactly similar... 

Ans. Primary, secondary, and tertiary alcohols are alcohols in which the OH is attached to a primary, secondary, and tertiary carbon, respectively. Recall that 1 °, 2 °, and 3 ° carbons are carbons having a total of one, two, and three bonds to other carbons. 1(a) and 11(b) are primary alcohols, Kb) is a secondary alcohol, and 11(a) is a tertiary alcohol. 

The obvious dilferences in the reactivity of primary, secondary, and tertiary carbon atoms are normally quite satisfactory to explain their selectivity. Esterification of alcohols invariably adopts the same sequence i.e., pri->sec->tert. In fact, the tertiary alcohols are generally quite rmreactive with regard to the esterification. 

T2, the spin-spin relaxation time. Carbons that are attached directly to hydrogen (primary, secondary, and tertiary carbons) experience strong interactions and decay more rapidly than carbons without attached hydrogen (quaternary carbons). When the decoupler is switched on after a time, tt, the resultant signal is due primarily to quaternary (nonprotonated) carbons. However, methyl groups (CH3) are not completely suppressed because of their rapid rotation in the solid state and appear in the aliphatic carbon signal. These carbons can be distinguished from the other quaternary aliphatic carbons on the basis of chemical shifts. 

Provided a branched alkane has only primary, secondary and tertiary carbon atoms, all hydrogen atoms can be exchanged by the a/S mechanism the carbon skeletons (devoid of hydrogen atoms) of some such molecules are shown in the first row of Table 6.5. The presence of a quaternary carbon atom as in neopentane (2,2-dimethylpropane) prevents the formation of an aa-diadsorbed species, so that multiple exchange, if it oecurs, must of necessity proceed through either aa- or ay-diasorbed structures, or where possible through anaS structure. Carbon skeletons of molecules of this type are shown in the lower part of Table 6.5, but... 

Several radical intermediate-based methodologies have been introduced for decarboxylations as an extension of earlier efforts on the deoxygenation of alcohols. One of the early approaches involved the TBTH reduction of the dihydrophenanthrene derivative of carboxylic acids (equation 71). However, the difficulties associated with the preparation of the phenanthrene ester derivatives and lack of success with tertiary carboxylic acids proved to be disadvantageous. A more notable discovery in this context is the demonstration that esters derived from COOH groups attached to primary, secondary and tertiary carbons and iV-hydroxypyridine-2-thione undergo efficient reductive decarboxylation in the presence of TBTH in refluxing benzene or toluene... 

Table S2.9 Average bond energies (kJ/mol) (Subscripts p, s and t indicate primary, secondary and tertiary carbon atoms 1 and 2 indicate the number of atoms of a given type, connected to the atom under consideration superscript indicate the element bonded to the polyvalent atom)...

The formation of PP macroradicals is an easy process initiated by more or less any radical initiator. It occurs spontaneously in oxidative processes. Alkyl radiccils (except for methyl) are usually not reactive enough to initiate an efficient macroradical formation in PP. Oxyl radicals, formed by a thermal decomposition of peroxides, are the most convenient species for crosslinking initiation. The transfer of the radical centre to PP is selective to a certain extent. At temperatures usual for peroxide decomposition, the ratio of the rate constants of the abstraction of hydrogen from primary, secondary, and tertiary carbon by the oxyl radical is approximately 1 3 10 [2]. 

The types of reaction open to a nitrene are broadly similar to those undergone by carbenes, but the reactivity of nitrenes is considerably lower. Nitrenes are less indiscriminate than carbenes in their reactions with primary, secondary, and tertiary carbon-hydrogen bonds, for instance, and nitrenes are also somewhat electrophilic, preferring an 0-H over a C-H bond. By far the most frequently used nitrene source is an aryl azide (see Table IV). The derived aryl nitrenes are much less reactive than a-keto, a-sulfonyl, or a-phosphoryl nitrenes, but the use of these acyl nitrenes is ruled out by the high chemical reactivity of the precursor species acyl azides, sulfonyl azides, and phosphoryl azides. 

FIGURE 2.34 The four kinds of butyl compounds used to illustrate the bonding in primary, secondary, and tertiary carbons (X is not carbon or hydrogen). 

---