Chapter 2: Alkanes and Cycloalkanes
Table of contents of main topics:
Nomenclature of alkanes and the IUPAC system
Alkanes are molecules that consist of carbons and hydrogens (hydrocarbons) and have only single carbon carbon bonds (saturated).
Going from structural formula to line angle formula.
Carbons are represented by vertices and hydrogens are not shown.
Naming Alkanes:
Find the longest continuous carbon chain (can be vertical, horizontal, etc). The name of the parent chain will start with a prefix signifying the number of carbons in the chain and end with "ane"
2.Number your parent chain from the end closest to the first substituent.
3. If there are multiple substituents in equivalent positions from both ends. Start numbering from the end that has the substituent with lowest alphabetical order.
Ethyl starts with e which is alphabetically before m (methyl). So we number from the right end in this case.
4. Once you numbered the parent chain, it is time to name the substituents (groups that come off of the parent chain.) Substituents are named with a number (identifying the carbon to which the substituent is attached) followed by name. Alkyl substituents are named with a prefix (for the number of carbons) followed by ending "yl"
5. If there are multiple substituents with the same name, use prefixes di, tri, tetra.
6. Put the name together with the name of substituents first followed by the name of the parent chain. Substituents must go in alphabetical order. The prefixes di, tri, tetra.. are not used in alphabetizing.
7. Where there are two or more parent chains of identical length, choose the parent
chain with the greater number of substituents.
Write the IUPAC and common names for these alkanes.
Constitutional Isomers
Constitutional isomers are molecules with the same molecular formula but different structure.
As you can see both molecules above have the same formula, C4H10. However, they have a different structure and connection. Their names would be different as well: butane versus 2-methylpropane. They are constitutional isomers.
Newman Projections
Newman projection is a short hand way to represent a molecule.
You can imagine standing in front of the molecule and looking at carbon 1 like the person on the top left. The carbon has group a on top as shown in the Newman projection, group b to the right and group c to the left. Behind carbon 1 is carbon 2. The person can't see it but can see the attachment that it has: t to the left, e to the left and d at the bottom. When the groups on the two carbons are spread out, the conformation is called staggered. When they are right behind one another, like the second molecule, the conformation is called eclipsed.
Staggered conformation is always more stable and lower in energy than eclipsed.
Chairs
Cyclohexane can adopt a lot of different conformations but there is one in particular that relieves both its angular and torsional strain. That conformation is called a chair.
Let's first learn how to draw a chair conformation.
Positions that are straight up or straight down are axial or those that are on a diagonal are equatorial.
It is also important to know which positions on the chair are up and which are down.
Practice: Draw the most stable chair conformation of cis -1,2- dimethyl cyclohexane and trans -1,2-dimethyl cyclohexane
Mayya's Tip
Draw the cyclohexane with the groups on it. Cis means both are either wedge or dash (choose either one).Trans means one is wedge and another is dash.
Draw the chair and number the carbons on it 1 through 6 where 1 is on the top right going clockwise. Add substituted to the chair. Wedge= UP and dash= DOWN. If not sure, look at the up and down positions from the guideline above.
Draw the flipped chair and give 1 to the bottom right carbon. Add substituents.
Label all substituents as axial or equatorial. The one with more equatorial groups or with bigger groups being equatorial is most stable/lowest energy.
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