Chapter 8. Organic Rxns 1.

Alkanes

Nomenclature

Systematic naming scheme for organic compounds -- IUPAC rules

1. Name the parent chain
choose the longest chain as the parent chain
2. Name the alkyl groups attached to the parent chain
You can check the table below for the names of alkyl groups
3. Determine the point of attachment of alkyl groups to the parent chain
Number the parent chain such that the branch alkyl group is attached to the smaller numbered carbon.
• Alphabetical order the alkyl groups with numbering specifying the position

# of C	Name	Group Formula
1	methyl
2	ethyl
3	propyl
4	butyl
5	pentyl
6	hexyl
branched groups
3	isopropyl
4	isobutyl
4	sec-butyl or s-butyl
4	tert-butyl or t-butyl

When naming:

• alkane --- replace the -yl of each of the group with -ane
• alkene --- replace the -yl of each of the group with -ene
• alkyne --- replace the -yl of each of the group with -yne

For example, four-carbon hydrocarbon alkane is: butane , and six-carbon is hexane .

Four-carbon chain hydrocarbon with a double bond is butene . Both are butene; notice the position of the double bond. One can have two double bonds with 4-carbon chain , we call this molecule butadiene..

Five-carbon chain with a triple bond is pentyne, . Also, different triple bond configurations would also be pentyne, .

Constitutional Isomers

When chemical formulas are the same, but atomic connections are different, such molecules are called constitutional isomers.

So, for example, the following two melecules are the constitutional isomers of C₅H₁₂:

pentane	2-methylbutane

Conformations

We learned that you can assign 3-dimensional shape associated with carbon atoms depending on the bonding situations. If the molecules have the same formula and the atomic connections are the same, but they differ in shape, is called conformers. For example, the following two structures are both butanes, but the have different conformations.

These two structures are converted back and forth with nearly free rotations.

Cycloalkanes

As we have seen in Chapter 4, there are many different structures we can make from a chemical formula. One of such was cyclic structures. In the following table, some cycloalkanes are listed:

Name	Lewis Structure	Skeletal Structure	Side view
cyclopropane
cyclobutane
cyclopentane
cyclohexane

IUPAC names of cyclic alkane is to have the same rule:

This compound is named as 1-ethyl-3-methylcyclohexane. Notice the numbering.

Alkenes, Alkynes, and Aromatic Compounds

Alkenes contain at least one double bond. Alkynes contain at least one triple bond.

Naming

Naming of alkene and alkyne is given such that the double bonded (or triply bonded) carbon gets prioritized. For example, is named as 4,5-dimethyl-2-hexene.

Aromatic ring is special cyclic alkene. Because of three double bonds, benzene is a very stable compound. The two compounds on the left side are equivalent. So, we generally combine them together to draw benzene to be what is represented on the right-most structure with a circle.

Compounds such as below are common:

Benzyl alcohol

Benzaldehyde, the smell of almonds

Benzoic acid

Geometric Isomers

When there is a double bond, the rotation along the C-C bond is no longer a free rotation. This creates two distinct geometric isomers. For example,

trans-2-pentene	cis-2-pentene
Notice the orientation of the functional groups along the double bond.

Reactions of Hydrocarbons

Alkanes

Halogenation

You can add halogen atom to alkane by using light, symbolized by $h\nu$. There is no specificity as to where the chlorine atom attaches.

Alkenes and Alkynes

Pt catalysis: hydrogenation of alkenes and alkynes

We've seen this in Chapter 4.

Markovnikov's rxn: HCl or HBr addition across double bonds

Alkenes and alkynes can be halogenated as well as hydration via Markonikov mechanism of protonation. The first step is to add H⁺, and this step is crucial in product distribution. The cation formed upon addition of H⁺ makes a cation on some carbon centered at less hydrogen attached in the original compounds.

Halogenation on benzene:

The following reaction is called a class of reaction, aromatic substitution reaction.

Carboxylic Acids

Naming of carboxylic acid is similar to what we have seen so far. For example, the following compound, is called 2,3-dimethylbutanoic acid.

Phenols

The structure of phenol is as follows:

Caroxylic Acids and Phenols as Weak Acids

Weak Acid = acid that doesn't dissociate completely. And, weak acid forms an equilibrium, HA ⇄ H⁺ + A^-

We have seen that acetic acid is a weak acid,

CH₃CO₂H ⇄ H⁺ + CH₃CO₂^-     K_a = 1.8 X 10^-5 or pK_a = 4.74

Phenol is weaker acid, and is given by the following:

    K_a = 1.1 X 10^-10 or pK_a = 9.96

Salts of weak organic acids are used as perservatives in the food industry. You've seen some of those in the list below:

Name	Structure	Parent Acid
Sodium benzoate		Benzoic acid
Potassium sorbate		Sorbic acid (hexa-2,4,dienoic acid is correct) or even 2,4-hexadienoic acid
Potassium p-bromophenoxide		p-bromophenol

Biological carboxylic acid

Depending on pH, the structure changes:

	pKa = 4.75	pH > 5 --deprotonated pH < 5 -- protonated
Palmitic acid

Acids used for chemical exfoliation: All are α hydroxy acids.

Formula	name	source	pKa
	Glycolic acid	sugarcane	3.83
	Lactic acid	sour milk	3.08
	Malic acid	apples and grapes	3.40
	Tartaric acid	wine	2.98
	Citric acid	citrus	3.18
	Salicylic acid	willow bark	2.97
	Trichloroacetic acid	chemical synthesis	0.70

Preparing Esters

Preparation of ester is done by adding carboxylic acid and alcohol with a help of acid. R¹-COOH + HO-R² ⇄ R¹-COO-R² + H₂O

Amines

1^° (primary) amine - one carbon next to N

2^° (secondary) amine - two carbons directly attacehed to N

3^° (tertiary) amine - three carbons directly attacehed to N

4^° (quaternary) amine - four carbons directly attacehed to N

Naming is done by the following way:

Type	Name	Formula
1°	Ethanamine
1°	propanamine
1°	butanamine
1°	pentanamine
2°	N-methyl-1-propanamine
2°	N-ethyl-1-propanamine
2°	N-methyl-1-propanamine
2°	N-methyl-1-butanamine
2°	N-methyl-1-pentanamine

Cyclic amines are very important in biological chemistry.

These cyclic amines are found in DNA, and these are call DNA bases.

Amines as Weak Base

Amine is a weak base. The amine nitrogen has a lone-pair electrons, which makes amine to accept H⁺. This is why amine is a weak base.

For example, methylamine, CH₃NH₂ form an equilibrium,

CH₃NH₂ + H₂O ⇄ CH₃NH₃⁺ + OH^- If the pH < pK_a, then it is protonated. If the pH > pK_a, then it is deprotonated.

The pK_a of aniline is 4.6. If pH is below 4.6, then the aniline is protonated, while pH > 4.6, aniline is unprotonated.

Amides

As we have seen amide has amine group next to C=O Hexanamide is the name for amide with 6-carbon chain. When the substitution exists on nitrogen, you can name as follows:

It can be synthesized by adding ammonia, NH₃, to carboxylic acid:

The reaction can be reversed by heating.

Arguably the most important reaction of the synthesis of amide is to make a primary chain of amino acids to make a protein. Amino acid structure in general is: Different amino acid is decided by the "side chain" labeled as R. Protein is made of a sequence of amino acid chain. The chain, called peptide, is made from reacting the carboxyl group of one amino acid to the amine group of another amino acid. The side chains, R₁ and R₂ may or may not be the same. In the naturally occuring amino acid, there are 20 or so different side chain exit. We'll talk about this little more in Chapter 12. The right side, where the carboxyl group is located, is called C terminus and the left side, where the amine group is located, is called N terminus.

Summary of Rxns in Chapter 8

Additional Questions on Rxns