Counting Carbon Atoms

There is a definite relationship between the peaks at (M + 1) and M that is directly related to the number of carbons present in hydrocarbon molecules  

This relationship is very valuable in characterizing an unknown compound. The calculation just presented is for hydrocarbons (compounds containing only carbon and hydrogen) and ignores the contribution from the isotope of hydrogen, called deuterium, due to its low relative abundance of 0.016%. In rigorous calculations, the contribution from deuterium would be taken into account. 

Isotope Mass Relative Abundance Mass Relative Abundance Mass Relative Abundance 

A list of common isotopes found in organic compounds and their relative abundance in nature is given in Table 10.4. A complete table of the relative abundance of natural isotopes for all elements is located in Appendix 10. A. As can be seen in Table 10.4, the natural abundance of deuterium is only 0.016% of the H abundance, so it can usually be ignored in calculations. 

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Carboxylic acids Change the -e of the parent hydrocarbon to -oic acid. To identify the parent acid, include the C atom of the —COOH group when counting carbon atoms. Thus, CH 3CH2CH2COOH is butanoic acid. 

Now that we know how to count carbon atoms, we must learn how to count the hydrogen atoms in a bond-line drawing of a molecule. Most hydrogen atoms are not shown, so bond-line drawings can be drawn very quickly. Hydrogen atoms connected to atoms other carbon (such as nitrogen or oxygen) must be drawn ... 

This doesn t count carbon atoms with single electrons (free radicals). You ve got to draw the line somewhere, and I ve chosen to eliminate the more radical elements. If you want to put them in, you can draw your own table. 

Note that, just as we counted the nails by weighing them, we have also developed a method of counting carbon atoms by weighing. For the nails, the technique was merely convenient. If we count one nail per second, it would take over seven hours to count... 

Counting Carbon Atoms To count carbon atoms by weighing, we need to know the mass of individual atoms, just as we needed to know the mass of the individual jelly beans. We learned from Chapter 3 that the atoms of a given element exist as isotopes. 

Now that we know the average mass of the carbon atom (12.01 amu), we can count carbon atoms by weighing samples of natural carbon. For example, what mass of natural carbon must we take to have 1000 carbon atoms present ... 

You will read more about the rules of balancing equations later in the chapter.) Thus, we begin by counting carbon atoms. 

Both schemes count five carbon atoms m their longest continuous chain and bear a methyl group as a substituent at the second carbon An alternative numbering sequence that begins at the other end of the chain is incorrect... 

Count the number of carbon atoms in the ring and the number in the largest substituent chain, (f the number of carbon atoms in the ring is equal to or greater than the number in the substituent, the compound is named as an alkyl-substituted cycloalkane. If the number of carbon atoms in the largest substituent is greater than the number in the ring, the compound is named as a cycloalkyl-substituted alkane. For example ... 

At its simplest, 13C NMR makes it possible to count the number of different carbon atoms in a molecule. Look at the l3C NMR spectra of methyl acetate and 1-pentanol shown previously in Figures 13.3b and 13.6b. In each case, a single sharp resonance line is observed for each different carbon atom. 

The information derived from 13C NMR spectroscopy is extraordinarily useful foT structure determination. Not only can we count the number of nonequivalent carbon atoms in a molecule, we can also get information about the electronic environment of each carbon and can even find how many protons each is attached to. As a result, we can answer many structural questions that go unanswered by TR spectroscopy or mass spectrometry. 

In this shorthand, they assume that any vertex between two lines contains a carbon atom unless specified otherwise. If there are several carbons in a row, they will make the lines join at angles, so that they can count the carbons if need be. 

To tell someone what we mean by l mol, we could give them 12 g of carbon-12 and invite them to count the atoms (Fig. E.l). Because counting atoms directly is impractical, we use an indirect route based on the mass of one atom. The mass of a carbon-12 atom has been found by mass spectrometry to be 1.992 65 X 10 23 g. It follows that the number of atoms in exactly 12 g of carbon-12 is... 

In the technique developed by Willard Libby in Chicago in the late 1940s, the proportion of carbon-14 in a sample is determined by monitoring the (1 radiation from C02 obtained by burning the sample. This procedure is illustrated in Example 17.4. In the modern version of the technique, which requires only a few milligrams of sample, the carbon atoms are converted into C ions by bombardment of the sample with cesium atoms. The C ions are then accelerated with electric fields, and the carbon isotopes are separated and counted with a mass spectrometer (Fig. 17.19). 

Carboxylic acids are named systematically by replacing the -e of the parent hydrocarbon by the suffix -oic acid the carbon atom of the carboxyl group is included in the count of atoms to determine the parent hydrocarbon molecule. Thus, formic acid is formally methanoic acid, and acetic acid is ethanoic acid. 

Aldehydes and ketones For aldehydes, identify the parent hydrocarbon include the C of—CHO in the count of carbon atoms. Then change the final -e of the hydrocarbon name to -al. The C in the —CHO group is always carbon l, at the end of a carbon chain, and is not explicitly numbered. For ketones, change the -e of the parent hydrocarbon to -one and number the chain in the order that gives the carbonyl group the lower number. Thus, CH3CH2CH2COCH3 is 2-pentanone. 

Double and triple bonds are counted as if they were split into two or three single bonds, respectively, as in the examples in Table 4.1 (note the treatment of the phenyl group). Note that in a C=C double bond, the two carbon atoms are each regarded as being connected to two carbon atoms and that one of the latter is counted as having three phantom substituents. 

The order of a sigmatropic rearrangement is expressed by two numbers set in brackets [ij]. These numbers can be determined by counting the atoms over which each end of the s bond has moved. Each of the original termini is given the number 1. Thus in the first example above, each terminus of the s bond has migrated from C-1 to C-3, so the order is [3,3]. In the second example, the carbon terminus has moved from C-1 to C-5, but the hydrogen terminus has not moved at all, so the order is [1,5]. 

It is a common mistake to forget that the ends of lines represent carbon atoms as well. For example, the following molecnle has six carbon atoms (make sure you can count them) ... 

EXERCISE 1.1 Count the number of carbon atoms in each of the following drawings ... 

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