Carbon - Structures and Functional Groups

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In CHEM 105, you likely were responsible for drawing many Lewis Structures that involved carbon. This is for a very good reason; carbon is central to many (and I really do mean many) molecules that are incredibly important for all divisions of science. For example, the field of energy has been dependent on carbon pretty much since its inception. Energy production in large relies on the combustion of carbon containing molecules. Much of the power we generate comes from the combustion of coal and our machinery run by combusting various forms of carbon chains (hydrocarbons).
Coal: readily combustible rock made up of at least 50% carbon by weight. Other elements present include hydrogen, oxygen, and sulfur.

Hydrocarbons: compound containing only carbon and hydrogen.
You've likely seen and drawn the structure of hydrocarbons many times. Get used to it - you're going to draw them a lot more by the end of your time in science. Here are a couple examples:

Skeletal structures follow the same rules as Lewis structures but carbon atoms are never shown.
Hydrogen atoms covalently bonded to carbon are not shown.
Bonds between carbon atoms are shown as a single line (just like in a normal Lewis structure)
Each carbon is represented by the end of a line.
Adjacent bonds are shown by changing the angle - this way the number of carbons can be determined.
The number of hydrogen atom on carbon is inferred by the octet rule
Heteroatoms (these are all other atoms) are always shown.
Hydrogen atoms covalently bonded to heteroatoms ARE always shown.

The thing is that nobody really wants to draw out the full structure of hydrocarbon chains - they can get REALLY long and tedious. As a solution, scientists have come up with a way to let us be a little bit lazy but still communicate the same information: skeletal structures. As the name implies, these structures only show the bare minimum of information that is needed to communicate exactly what the structure looks like. Here are the rules for drawing them:
Let's walk through this a couple times together and then you can try a couple on your own.
CH3CH2CH2CH2CH3 - the complete Lewis structure is shown here (expanded structure):

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We could also show this in a condensed form (condensed structure) where the C-H bonds are not explicitly shown:
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Note that there are 5 carbons linked together through single bonds. The skeleton structure shows the 4 C-C bonds. The two ends of the structure are carbon atoms and the 3 points are also carbon atoms. We don't show any of the hydrogen atoms because they are all bound to carbon.

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Now let's start adding in some double and triple bonds. Follow the strategy we used in the last problem: add carbons and then consider octets.
Untitled picture.png CH
/ CH
O 一 O
CH3

Moving right along - let's start adding some heteroatoms. Consider the condensed structure. The process of making a skeletal structure is identical to what we saw above; however now we need to make sure to show the heteroatoms (with lone pairs!)
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он
сн
сн

Carbons are said to be "saturated" with hydrogen or other heteroatoms.
All carbons have sp3 hybridization and tetrahedral geometry.
Alkanes - only contain C-C single bonds.
The hydrocarbon is now "unsaturated".
At least two carbons have sp2 hybridization and trigonal planar geometry.
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Can be "cis" (left side of figure below) or "trans" (right side) depending on how groups are arranged around the double bond. Note that in the cis conformation, both carbons come off the double bond on the same side (cis = same) and opposite sides in the trans conformation (trans = opposite).
Alkenes - at least one C-C double bond is present
The hydrocarbon is now "unsaturated".
At least two carbons have sp hybridization and linear geometry.
Alkynes do not have cis or trans configurations because there are only two atoms attached to the sp hybrizided carbons.
The skeletal structures of alkynes are drawn so that the linear geometry is shown. Here, there are three carbons: two at the ends and a central carbon where the bond pattern changes.
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1Alkynes - at least one C-C triple bond is present
Aromatics - a special type of hydrocarbon where the carbons are arranged in a ring with each carbon having an sp3 hybridization. Consequently, each carbon has one double bond and two single bonds. The simplest aromatic compound is benzene (C6H6) and is shown below.
These hydrocarbons can be broken up into classes based on the types of bonds that are present:
Hydrocarbon Classifications

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Aromatics - a special type of hydrocarbon where the carbons are arranged in a ring with each carbon having an sp3 hybridization. Consequently, each carbon has one double bond and two single bonds. The simplest aromatic compound is benzene (C6H6) and is shown below.

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These compounds are very important and very stable due to the resonance patterns that are inherently a part of their chemistry.
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Alkanes: In CHEM 105, you probably learned ways to name molecules that contain only carbon and hydrogen. For example, C2H6 would be dicarbon hexahydride. However, this name limits what we can learn about the actual structure of the compound. Consequently, more thematic names have been assigned to the common alkanes. These names are based on the length of the carbon chain. The table below summarizes the names for the first ten hydrocarbons.

Number of Carbons
Molecular Formula
Name
1
CH4
methane
2
C2H6
ethane
3
C3H8
propane
4
C4H10
butane
5
C5H12
pentane
6
C6H14
hexane
7
C7H16
heptane
8
C8H18
octane
9
C9H20
nonane
10
C10H22
decane

The carbon chain needs to be numbered in an way that a terminal carbon is "1". This does not necessarily need to be the first carbon drawn.
Some hydrocarbons have atoms or groups that replace one or more hydrogens - these are called functional groups because they influence the function of a compound by altering the chemical and physical properties - more on this below. Using halide and alkyl substitutions as an example, we can establish some very simple rules that let us name these compounds. A few general rules apply when naming substituted alkanes:
Hydrocarbon Nomenclature

The carbon chain needs to be numbered in an way that a terminal carbon is "1". This does not necessarily need to be the first carbon drawn.
The substitution and its location on the carbon chain gets identified before naming the hydrocarbon chain.
The carbon number should be as small as possible.
For halides, drop the -ide and add -o (e.g. fluoro or bromo)
For alkyl substitutions, use the table below.

An example will help clarify this.

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Here the parent chain is five carbons (pentane) and the substitution is a chlorine (which become chloro). We could number this carbon chain starting from the left or right. When numbering from the left, we see that the corresponding name is 4-chloropentane, while starting the numbers at the right makes it 2-cholorpentane. Since 2 < 4, the 2-chloropentane is the correct name.

Untitled picture.png 4-chloropentane
2-chlorpentane

The substitution can also be another carbon chain.
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In this case, the first thing we need to do is identify the longest carbon chain. Note that it is not always the most obvious. For this molecule, the longest chain is seven carbons, not the six you might expect at first glance. So the base name of this compound will be heptane. The substitution is a -CH3 (methane) group. To name the compound, we need to drop the -ane and add -yl; the functional group is a methyl. The name of this compound is 3-methylheptane.
Untitled picture.png 3-methylheptane

Any carbon chain length can be a functional group. All you need to do is drop the -ane and add -yl to use it in the compound name.
Ink Drawings
Ink Drawings

Any carbon chain length can be a functional group. All you need to do is drop the -ane and add -yl to use it in the compound name.
Hydrocarbon Name
Functional Group Name
methane
methyl
ethane
ethyl
propane
propyl
butane
butyl
pentane
pentyl
hexane
hexyl
heptane
heptyl
octane
octyl
nonane
nonyl
decane
decyl

Try this one:
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The longest chain is 8 carbons and a propyl group is attached to the 4th carbon: 4-propyloctane.

Alkenes: Simple alkene names derive from the corresponding alkane. Consider the molecule below:
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This hydrocarbon has four carbons, so the parent name is butane. We indicate that there is a double bond by changing the "-ane" to "-ene". This molecule is named butene, which tells us that it is a four carbon alkene. However, the molecule shown just below is also a 4 carbon alkene that we could name butene.
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Since these are not actually the same molecule, we MUST have a way to differentiate them. We do this by indicating the position of the double bond. If we number the four carbons from left to right as seen below, we see that position of the double bond is on the 2nd carbon in the first molecule, so it is named 2-butene, and the first carbon in the last compound (1-butene).
Untitled picture.png 2-butene
1 -butene
As we saw above, the numbering can begin from either end. As with naming alkanes, the number designator is as small as possible. Consider the following example:
Untitled picture.png 1 -butene
3-butene

Untitled picture.png 1 -butene
3-butene
These two identical molecules have been numbered from the right or left. When numbering starts on the right side, the numbering suggests that the compound is named 3-butene; however, if the numbers start with the last carbon, then the double bond begins on the first carbon and we name it 1-butene. So…numbering from the right to left produces a more correct name. Consequently, 1-butene can be drawn two ways (see above two images) but represent the same molecule no matter how it is drawn.

Here are a couple more examples:
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This 6 carbon alkene has the base name hexene (hex = 6, ene = double bond). Starting the numbers from the right makes it 2-hexene.

Alkynes: The process for naming an alkyne is identical to naming an alkene. The only difference is that you replace -ane with -yne.
Untitled picture.png 1 -propyne
1 -propyne
2-butyne
2-pentyne

Isomers

In many cases, different compounds can have the same molecular formula. These compounds are called isomers. Isomers fall into two categories: structural and spatial.

Structural isomers differ by the placement of a substitution. For example, 1-bromobutane and 2-bromobutane have identical chemical formulas (C4H9Br) but bromine is present on different carbons. These two compounds are structural isomers
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Spatial isomers, also called stereoisomers, have the same bonding structure but the arrangement of functional groups on a carbon are different. In many cases, stereoisomers are only identifiable when the 3-dimensional structure of a compound is considered (this is covered in detail in Organic Chemistry). 2D stereoisomers also exist; in fact, we have already seen an example of spatial isomers. When discussing alkenes, we introduced the cis and trans conformations. These compounds have identical chemical formulas but differ by the arrangement of functional groups around the double bond.
Untitled picture.png 2-butene (cis)
2-butene (trans)

Functional Groups

The concept of functional groups was briefly introduced above. The idea is that by adding other atoms to a carbon chain, the chemical and physical properties change - sometime very significantly. In this section, we will be introducing common functional groups and focusing on how to recognize them and to draw them. A little bit later in the class we will be looking at some of the basic chemistry that these functional groups can undergo. Below is a table that summarizes the groups that you need to be able to recognize and draw.

Untitled picture.png Alcohol
Amine
H3C — OH
H
C — NH
2
Ether
Ester
•O.
Ketone
CH
3
CH
3
CH
H3Cs...% 3
•O.
Amide
Carboxylic Acid
OH
Aldehyde
•O.
Each of these functional groups can be incorporated into a carbon chain at pretty much any position. The resulting structure will have different properties than the parent hydrocarbon. For example, consider the three compounds shown below. Each are based on a hexane carbon chain but are functionalized differently. The middle compound is an alcohol that will be more soluble in water than hexane and the last is a carboxylic acid that will also be more soluble but will additionally make the solution acidic.
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