The a-Carbon Atom and Its Hydrogens

It is convenient to use the Greek letters a, (3, y, and so forth, to locate the carbons in a molecule in relation to the carbonyl group. The carbon atom adjacent to the carbonyl is the a-carbon atom, the next one down the chain is the (3 carbon, and so on. Butanal, for example, has an a carbon, a (3 carbon, and a y carbon. 

Carbonyl group is reference point no Greek letter assigned to it. 

PROBLEM 18.1 How many a hydrogens are there in each of the following  

SAMPLE SOLUTION (a) This ketone has two different a carbons, but only one of them has hydrogen substituents. There are three equivalent a hydrogens. The other nine hydrogens are attached to p-carbon atoms. 

Other than nucleophilic addition to the carbonyl group, the most important reactions of aldehydes and ketones involve substitution of an a hydrogen. A particularly well studied example is halogenation of aldehydes and ketones. 

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Stearic acid is a member of the group called fatty acids. These are hydrocarbon chains (chains made of repeated units of a carbon atom and two hydrogen atoms) with a carboxyl group at one end. A carboxyl group is the COOH in the chemical formula. It is what turns the hydrocarbon chain into an organic acid. 

At first sight formaldehyde (methanal, CH2=0) seems the ideal electrophilic partner in a mixed aldol reaction. It cannot enolize, (Usually we are concerned with oc hydrogen atoms in an aldehyde. Formaldehyde does not even have a carbon atoms.) And it is a super aldehyde. Aldehydes are more electrophilic than ketones because a hydrogen atom replaces one of the alkyl groups. Formaldehyde has two hydrogen atoms. 

The general formnla for cyclic cycloalkanes is C H2 . Fignre 24.6 gives the names and strnctnral formnlas for the first fonr members of the cycloalkane series. In the condensed strnctnral formnlas, a carbon atom and its attached hydrogen atoms are assnmed to be at each comer. 

Figure 15.3 The free-energy diagram for an electrophilic aromatic substitution reaction. The arenium ion is a true intermediate lying between transition states 1 and 2. In transition state 1 the bond between the electrophile and one carbon atom of the benzene ring is only partially formed. In transition state 2 the bond between the same benzene carbon atom and its hydrogen atom is partially broken. The bond between the hydrogen atom and the conjugate base is partially formed.

Ethylene, C2H4, is a molecule. We can represent it as shown in Fig. 22.1(a), where the square box is a carbon atom, and the small circles are hydrogen. Polymerisation breaks... 

The carbon atom we have added must form three additional bonds to satisfy its tetravalent bonding capacity. If all of these bonds were to hydrogen atoms, we would have the completed molecule CH3OH, and would also have two hydrogen atoms and a carbon atom left over. Therefore, one of the bonds our first carbon atom forms must be to the other carbon atom, and the two other bonds must be to hydrogen atoms. We have then... 

Molecules vary considerably in complexity. Molecular oxygen is made up of two oxygen atoms, so its chemical formula is O2. A carbon monoxide molecule contains one atom of carbon and one atom of oxygen, so its chemical formula is CO. Each molecule of methane, the major constituent of natural gas, contains one carbon atom and four hydrogen atoms, so its formula is C H4. You will encounter still more complicated structures, such as methanol (three different elements, C H4 O) as you progress through this book. 

Consideration of the feasibility of these shifts as concerted processes, i.e. via cyclic transition states, requires as usual a consideration of the symmetry of the orbitals involved. A model related to the transition state can be constructed by the device of assuming that the C—H a bond that is migrating can be broken down into a hydrogen Is orbital and a carbon 2p orbital. For the case where x = 1 in (36), the T.S. can then be considered as being made up from a pentadienyl radical (38), with a hydrogen atom (one electron in a Is orbital) migrating between the terminal carbon atoms of its Site system (i.e. a 6e system overall is involved) ... 

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