Secondary Carbon

1. Secondary Carbocation
2. Secondary Carbon Definition
3. Secondary Carbon Atom
4. Secondary Carbon Sn1 Or Sn2

Learning Objectives

Secondary Carbocation

1. Give two major types of reactions of alcohols.
2. Describe the result of the oxidation of a primary alcohol.
3. Describe the result of the oxidation of a secondary alcohol.

Chemical reactions in alcohols occur mainly at the functional group, but some involve hydrogen atoms attached to the OH-bearing carbon atom or to an adjacent carbon atom. Of the three major kinds of alcohol reactions, which are summarized in Figure (PageIndex{1}), two—dehydration and oxidation—are considered here. The third reaction type—esterification—is covered elsewhere.

Dehydration

The secondary organic carbon (SOC) estimated by the SOA-tracer method averaged 3.07 mgCm 3 in summer and 2.00 mgCm 3 in fall-winter, contributing 38.4% and 8.7% to OC, respectively. During summer, aromatics-SOC and isoprene-SOC reached 2.25 1.5 mgCm 3 and 0.64 0.7 mgCm 3 and accounted for 76% and 18% of the. Regrowing forests are absorbing just a small proportion of the carbon dioxide released from widespread deforestation in the Amazon, according to new evidence. Secondary forests—areas of new forest. Secondary Carbon Structural formula of propane (secondary carbon is highlighted red): A secondary carbon is a carbon atom bound to two other carbon atoms. For this reason, secondary carbon atoms are found in all hydrocarbons having at least three carbon atoms. In short, these definitions are assigned to carbon atoms based on the number of other carbon atoms they are connected to: Primary carbons are connected to one carbon only. Secondary carbons are connected to two carbon atoms. Tertiary carbons are. Secondary carbocations. In a secondary (2°) carbocation, the carbon with the positive charge is attached to two other alkyl groups, which may be the same or different. Examples: A secondary carbocation has the general formula shown in the box. R and R' represent alkyl groups which may be the same or different. Tertiary carbocations.

As noted in Figure (PageIndex{1}), an alcohol undergoes dehydration in the presence of a catalyst to form an alkene and water. The reaction removes the OH group from the alcohol carbon atom and a hydrogen atom from an adjacent carbon atom in the same molecule:

Under the proper conditions, it is possible for the dehydration to occur between two alcohol molecules. The entire OH group of one molecule and only the hydrogen atom of the OH group of the second molecule are removed. The two ethyl groups attached to an oxygen atom form an ether molecule.

(Ethers are discussed in elsewhere) Thus, depending on conditions, one can prepare either alkenes or ethers by the dehydration of alcohols.

Both dehydration and hydration reactions occur continuously in cellular metabolism, with enzymes serving as catalysts and at a temperature of about 37°C. The following reaction occurs in the 'Embden–Meyerhof' pathway

Although the participating compounds are complex, the reaction is the same: elimination of water from the starting material. The idea is that if you know the chemistry of a particular functional group, you know the chemistry of hundreds of different compounds.

Oxidation

Primary and secondary alcohols are readily oxidized. We saw earlier how methanol and ethanol are oxidized by liver enzymes to form aldehydes. Because a variety of oxidizing agents can bring about oxidation, we can indicate an oxidizing agent without specifying a particular one by writing an equation with the symbol [O] above the arrow. For example, we write the oxidation of ethanol—a primary alcohol—to form acetaldehyde—an aldehyde—as follows:

We shall see that aldehydes are even more easily oxidized than alcohols and yield carboxylic acids. Secondary alcohols are oxidized to ketones. The oxidation of isopropyl alcohol by potassium dichromate (K₂Cr₂O₇) gives acetone, the simplest ketone:

Unlike aldehydes, ketones are relatively resistant to further oxidation, so no special precautions are required to isolate them as they form. Note that in oxidation of both primary (RCH₂OH) and secondary (R₂CHOH) alcohols, two hydrogen atoms are removed from the alcohol molecule, one from the OH group and other from the carbon atom that bears the OH group.

These reactions can also be carried out in the laboratory with chemical oxidizing agents. One such oxidizing agent is potassium dichromate. The balanced equation (showing only the species involved in the reaction) in this case is as follows:

Alcohol oxidation is important in living organisms. Enzyme-controlled oxidation reactions provide the energy cells need to do useful work. One step in the metabolism of carbohydrates involves the oxidation of the secondary alcohol group in isocitric acid to a ketone group:

The overall type of reaction is the same as that in the conversion of isopropyl alcohol to acetone.

Tertiary alcohols (R₃COH) are resistant to oxidation because the carbon atom that carries the OH group does not have a hydrogen atom attached but is instead bonded to other carbon atoms. The oxidation reactions we have described involve the formation of a carbon-to-oxygen double bond. Thus, the carbon atom bearing the OH group must be able to release one of its attached atoms to form the double bond. The carbon-to-hydrogen bonding is easily broken under oxidative conditions, but carbon-to-carbon bonds are not. Therefore tertiary alcohols are not easily oxidized.

Example (PageIndex{1})

Write an equation for the oxidation of each alcohol. Use [O] above the arrow to indicate an oxidizing agent. If no reaction occurs, write “no reaction” after the arrow.

1. CH₃CH₂CH₂CH₂CH₂OH

Solution

The first step is to recognize the class of each alcohol as primary, secondary, or tertiary.

1. This alcohol has the OH group on a carbon atom that is attached to only one other carbon atom, so it is a primary alcohol. Oxidation forms first an aldehyde and further oxidation forms a carboxylic acid.
2. This alcohol has the OH group on a carbon atom that is attached to three other carbon atoms, so it is a tertiary alcohol. No reaction occurs.
3. This alcohol has the OH group on a carbon atom that is attached to two other carbon atoms, so it is a secondary alcohol; oxidation gives a ketone.

Exercise (PageIndex{1})

Write an equation for the oxidation of each alcohol. Use [O] above the arrow to indicate an oxidizing agent. If no reaction occurs, write “no reaction” after the arrow.

Summary

Alcohols can be dehydrated to form either alkenes (higher temperature, excess acid) or ethers (lower temperature, excess alcohol). Primary alcohols are oxidized to form aldehydes. Secondary alcohols are oxidized to form ketones. Tertiary alcohols are not readily oxidized.

Concept Review Exercises

1. In a reaction, compound W with the molecular formula C₄H₁₀O is converted to compound X with the formula C₄H₈O. Is W oxidized, reduced, dehydrated, or none of these? Explain.

2. In a reaction, 2 mol of compound Y with the molecular formula C₄H₁₀O is converted to 1 mol of compound Z with the formula C₈H₁₈O. Is Y oxidized, reduced, or neither? Explain.

Answers

1. neither; water is removed

Exercises

1. Name the three major types of chemical reactions of alcohols.

2. Why do tertiary alcohols not undergo oxidation? Can a tertiary alcohol undergo dehydration?

3. Draw the structure of the product for each reaction.

4. Draw the structure of the product for each reaction.

5. Write an equation for the dehydration of 2-propanol to yield each compound type.

   1. an alkene
   2. an ether

6. Draw the structure of the alkene formed by the dehydration of cyclohexanol.

Answers

5. 1. (mathrm{CH_3CHOHCH_3underset{180^circ C,: excess: acid}{xrightarrow{conc: H_2SO_4}} CH_3COCH_3+H_2O})
   2. (mathrm{2CH_3CHOHCH_3 underset{180^circ C,: excess: acid}{xrightarrow{conc: H_2SO_4}}(CH_3)_2CHOCH(CH_3)_2+H_2O})

Home> Org/Bio Chem > Terms: Primary, etc | Contact

Introductory Organic and Biochemistry

Terms: Primary, Secondary, Tertiary, Quaternary

The following table illustrates this:

Ouellette 2/e, p 71.

Methane, CH₄, is a special case. The C in methane is attached to zero other C. This C is sometimes called primary, for convenience. Alternatively, it can just be considered separate from any of these categories.

Carbocations

A carbocation (C atom carrying a positive charge) is classified the same way as a regular (neutral) carbon atom, above. That is, if the C of the carbocation is attached to one other C, then it is called a primary carbocation, etc.

The book discusses this in the context of addition reactions of alkenes; acid-catalyzed addition of HX proceeds through a carbocation intermediate. Ouellette 2/e p 120.

The same classification scheme is used for C atoms carrying a negative charge (carbanions) or carrying an unpaired electron (free radicals). Carbanions and carbon free radicals are not encountered in this course (though they are mentioned by Ouellette 2/e, in topics that we will not cover).

Alcohols

An alcohol is classified based on the C atom to which the -OH is attached. That is, if the alcohol -OH group is on a primary C, then the alcohol is a primary alcohol. Example: CH₃CH₂OH (ethanol) is a primary alcohol.

Hopefully, the classification of alcohols seems logical, following from the way C atoms are classified. But caution... the next time we use these terms is for amines, and that is a quite different story.

Ouellette 2/e p 219.

Alkyl halides are classified the same way as alcohols, discussed above. For example, CH₃CH₂Cl (chloroethane or ethyl chloride) is a primary alkyl halide, because the Cl is attached to a primary C. Ouellette 2/e briefly mentions this, p 204.

Amines

An amine is classified based on its N atom. If the N atom has 1 C attached, then the amine is primary, etc. Example: CH₃NH₂ (methylamine) is a primary amine, because the N has 1 C attached. The nature of the C itself is not relevant.

Note that the classification of amines is not done by the same general procedure as the others discussed above. Amines are not classified by their C atoms, but rather by the N atom.

The difference between how alcohols and amines are classified is made clear by comparing two compounds that look rather similar:

Ouellette 2/e p 291.

Amides and ammonium ions are classified the same way as amines. That is, groups based on amines are classified the same way as the amines.

Ouellette 2/e shows the classification of amides on pp 331 & 392.

Note that it is possible to have quaternary ammonium ions, but not quaternary amines. Ouellette 2/e introduces this, p 405. The tetramethylammonium ion is a quaternary ammonium ion. Choline, an important molecule in biochemistry, is also a quaternary ammonium ion; see Ouellette 2/e pp 349 & 377.

Protein structure

The same terms (primary, etc) are used to describe aspects of protein structure. These meanings have no relationship at all to the meanings above; they have no relationship to counting anything.

The term primary structure refers to the amino acid sequence in a protein.

The terms secondary and tertiary structure refer to the three-dimensional (3D) conformation of a protein chain. Secondary structure refers to the interactions of the backbone chain (that is, the amide linkages). Tertiary structure refers to interactions of the side chains. The distinction between secondary and tertiary is not entirely clear, and you will see various definitions. Emphasize understanding typical examples of secondary and tertiary structure, and let definitions emerge from those examples. Formal definitions are not so helpful here, unless you already understand the basic ideas.

Quaternary structure refers to the interaction between separate chains in a multi-chain protein. The types of interactions may be any of those found in secondary and tertiary structures.

Secondary Carbon Definition

Ouellette 2/e discusses the levels of protein structure in Sect 15.8.

Nucleic acids

The terms primary, etc., are used in essentially the same way for nucleic acids as for proteins, discussed in the previous section. Ouellette 2/e introduces this usage on p 452

Secondary Carbon Atom

Summary

The following table briefly summarizes all of the usages of the terms primary (etc) discussed above. These usages fall into three general categories. Terms shown in bold are section headings above.

Secondary Carbon Sn1 Or Sn2

Last update: February 15, 2019