Naming Organic Compounds
Naming organic compounds serves a basis for shared discourse amongst scientists. Standardization has been one of the most important tools in the advancement of the human race. IUPAC created a rigorous of naming chemical compounds to replace the archaic and piecemeal system that has persisted since antiquity. Some older names still remain though.
1. Naming Alkanes
IUPAC has provided standard naming conventions for the naming unbranched alkane chains. They are as follows:
| prefix | number of carbon atoms |
| meth(ane) | 1 |
| eth(ane) | 2 |
| prop(ane) | 3 |
| but(ane) | 4 |
| pent(ane) | 5 |
| hex(ane) | 6 |
| hept(ane) | 7 |
| oct(ane) | 8 |
| non(ane) | 9 |
| dec(ane) | 10 |
x. Reaction Mechanisms
Two of the most basic reaction types you will come across are substitution and elimination reactions. These, however, may proceed by quite different mechanisms. Understanding the underlying mechanism of a chemical reaction gives a deep insight into the nature of the atomic universe. It also allows us to better understand reactions that are more complex and don’t adhere to one of these simple mechanisms.
It will initially be difficult to make sense of the following table. Practice and notecards may help. There are ways to determine whether elimination or substitution will predominate. Heat generally favors elimination reactions whereas room temperature or cooled reactions usually allow substitution to dominate. Resonance stabilization better delocalizes charges than proximal carbons. As a result, resonance stabilized primary carbons generally quickly form carbocations and undergo substitution. It is difficult to get such molecules to under elimination because that destroys the inherent resonance stability. Recall that resonance requires carbons to be arranged in an alternating double bond/single bond manner.
| Substrate | Protic Solvent | Aprotic Solvent | Strong Base | Bulky Base |
| $H_2O$, MeOH, EtOH | DMSO, DMF (Strong Nucleophiles will encourage SN2: $I^-$, $RS^-$, $CN^-$) | $OH^-$, $CH_3O^-$, $EtO^-$ | $tBut-OH$ | |
| $CH_3-X$ | SN2 | SN2 | SN2 | SN2 |
| $CRH_2-X$ | SN2 | SN2 | SN2/E2 | E2 |
| $CRR’H-X$ | SN1/E1/SN2 | SN2 | E2 | E2 |
| $CRR’R”-X$ | SN1/E1 | SN1/E1 | E2 | E2 |
| $Allyl-CH_2-C$ | SN1 | SN1 | SN1 | SN1 |
| $Benzyl-CH_2-C$ | SN1 | SN1 | SN1 | SN1 |
Worked Practice Problems
1. Draw a structure for 2,3-dimethylhexane.
2. Draw a structure for 2,2-dimethylhexan-1-ol.
3. Draw a structure for 1-chloro-1-ethylcyclobutane.
4. In questions 1 through 3, which (if any) of the compounds had chiral carbons? Redraw the results in an R configuration below.
Only 2,3-dimethylhexane has a chiral center. We will now draw 3R-2,3-dimethylhexane. The simplest way to draw a chiral compound will be to identify the lowest priority group and send it going backwards into the page. Next, we put the priority 1 group in the top left, the priority 2 to the right, and so on. We then verify our drawing. If we had made a mistake swapping any pair of groups would reverse the chirality.
5. S-2-Bromobutane is dissolved in acetone with sodium cyanate. What is the major product? What is the reaction mechanism?
$S-2-bromobutane \xrightarrow{NaCN,CH_2O} R-2-butanenitrile$
Below, we see two versions to the reaction mechanism. The first is a better description, the second is considered acceptable by most teachers.
6. T-butylchloride is dissolved in water and heated. By what mechanism does this reaction proceed?
There’s a tertiary carbon so this will proceed via formation of a carbocation. We recall that heat favors elimination reactions so the major product should the result of elimination.
$T-butylchloride \xrightarrow{\Delta , H_2O} 2-methylpropene$
7. S-3-Bromo-3-methylhexane is reacted at room temperature in methanol. What reaction mechanism(s) will dominate? What are all possible products?
E1/SN1 will both be seen. SN1 will likely be favored, but E1 products will also be present.
8. Given the reactant depicted to the right, is elimination or substitution more likely? Why? Suppose, we conduct the reaction using tButOH. What products will be formed and why?
Elimination will almost surely dominate because that will create a highly resonance stabilized structure.
We expect only the elimination product here because we have a bulky base approaching a sterically hindered carbon. Moreover, we find that we will have one geometric isomer present. Let’s examine the newman project to understand why:
The antistaggered configuration of benzyl substituents is by far the most stable conformer. As a result, the reaction will proceed by extracting a hydrogen and kicking out one of the bromines. We notice this results in only the formation of the geometric isomer with the benzyl groups in a trans configuration.
9. Benzyl bromide is added to hydrosulfuric acid in DMSO. What are the products and what is the likely reaction mechanism?
The reaction goes by the SN2 mechanism (we have a relatively unhindered primary, though benzylic, carbon in an aprotic polar solvent) and produces benzyl thiol and benzyl disulfide. The first product will attack the reactant to form the disulfide in a second SN2 reaction.
10. Bromomethoxymethane is reacted in ethanol. What are the products? What is the reaction mechanism?
This reaction produces (methoxymethoxy)ethane via an SN2 substitution. SN2 will be heavily favored because we have a primary carbon, no strong base, and no indication of heat. The solvent here acts a reactant.
12. Complete the reaction schema to the right with the expected major product.
13. Complete the reaction schema to the right with the expected major product.
Given the NMR spectrum to the right, identify the compound. The is a triplet near 1 ppm and septet near 1.5 ppm. The compound is an alkane with small number of carbons. The peak integration yields a ratio of 1:3 hydrogens where 1 refers to the septet and 3 to the triplet.
The molecule is $CH_3^a – CH_2^b – CH_3^a$. The a superscripts are represented by the doublet and b superscripted hydrogens are represented by the septet.
