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Alkenes, Alkynes and Aromatic Structures

+Unsaturated Hydrocarbons

- consist of carbon based molecules with multiple bonds between the carbons

Alkenes- functional group that is characterized by C-C double bonds

Alkynes- functional group characterized by C-C triple bonds

Aromatics- structures that contain a benzene ring

+Characteristics of Alkenes

Alkenes have a hybridization of sp2, a molecular geometry of trigonal planar and a bond angle of roughly 120°

General formula:

They hybridization allows for 3 sp2 orbitals and 1 p orbital, which is why the carbon is still able to have 4 bonds.

They are hydrocarbons, so they are nonpolar in nature.

Structural isomers exist (same chemical formula but different arrangement of atoms).

Geometric isomers also exist (more on this later.)

+Naming Alkenes

Pretty similar to alkanes…but,

The name of the longest carbon chain ends in –ene.

You start numbering the carbons in the longest chain closest to the double bond.

You must indicate where the double bond is in the compound, so you use the lowest numbered carbon that is attached to the double bond.

Example: 2-hexene or hex-2-ene

+Branches off of alkene chains

Most are the same as alkanes

Chloro

Bromo

Iodo

Methyl

Ethyl

Propyl

If you have a C2H3 – group off of the main chain, it is called a vinyl group.

An allyl group

+Practice Naming and Drawing Alkenes

1) 3-hexene

2) 4-ethyl-2-heptene

3) 3,4-dimethyl-2-pentene

4) 2-cyclohexyl-3-hexene

5)

6)

+Multiple Double Bonds in Alkenes

Many compounds contain two or more double bonds and are known as alkadienes, alkatrienes, alkatetraenes, and so on, the suffix denoting the number of double bonds. The location of each double bond is specified by appropriate numbers, as illustrated to the right:

1,2-butadiene

1,2,3-butatriene

+Geometric Isomers Geometric isomers are

present in substances that have double bonds because of the nature of the double bond.

Single bonds allow free rotation around the bond. However, double bonds have more of a rigid/locked formation so rotation is not free…it is restricted.

This restricted nature comes from the fact that not only are sigma bonds involved, but p-orbital overlap occurs above and below the sigma bond.

These isomers are called either cis- or trans- depending on the location of groups branching off the double bond carbons.

These isomers are not superimposable, meaning that they have different arrangements in space when placed on top of one another.

They also have different physical properties as well.

+Cis- vs. Trans-

The green atoms are Cl.

You can see how the groups are called cis- if they are on the same, or adjacent side of the molecule.

They are called trans- if they are on opposite sides of the molecule.

This is works for other groups/branches that come off of double bonds.

+Practice

Draw these structures:

1) trans-3-heptene

2) cis-5-chloro-2-hexene

Orient the cis/trans designation of the longest carbon chain that surrounds(includes) the double bond.

Identify these as cis/trans:

1)

2)

+Cycloalkenes

These are cyclic compounds that contain a C-C double bond in the ring.

Most common cycloalkenes:

Cyclopentene

Cyclohexene

The carbons of the double bond are numbered 1 and 2, so no numbers are needed.

Other branches are numbered with the lowest number possible, after numbering the carbons of the double bond first.

Practice:

+Preparation of Alkenes

Most are prepared from saturated hydrocarbons.

Cracking- process in which saturated hydrocarbons are heated to very high temps.

In the presence of a catalyst (usually silica-alumina), the hydrocarbon breaks into smaller molecules. Some eliminate hydrogen to form alkenes.

Results in mixtures of hydrocarbons…

Dehydration of Alcohols-

Water is removed from an alcohol molecule (characterized by an OH group) in the presence of concentrated sulfuric acid.

See below…

+Physical Properties of Alkenes

Boiling points are slightly lower than corresponding alkanes.

Chains with 4 or fewer carbons are gases at room temp.

5-17 C chain length are liquids.

Higher than 17, usually a solid.

Densities are less than water.

Melting points of isomers are wildly different than boiling points.

This is because the structure of the isomer would affect the crystal structure.

+Chemical Reactions involving Alkenes

Alkenes are more reactive than alkanes, mostly because there are fewer than 4 bonds to C in alkenes and alkynes.

Addition Reaction- most common

Hydrogen, Halogens, Hydrogen halides, sulfuric acid and water are some substances that can be added to alkenes.

Can be converted to saturated molecules (alkanes) by addition with hydrogen.

A catalyst like nickel or platinum is required

+Alkenes and Addition Reactions Alkenes can be added to elemental halogens like

bromine and chlorine to produce substituted alkanes.

If concentrated sulfuric acid is added, the product will produce a hydrogen sulfate, which conforms to an alcohol when reacted with water.

+Alkenes plus Unsymmetrical Halogens

Alkenes can have an unsymmetrical halogen compounds added, like HCl or HBr. However, you would expect a mixture of products. According to a reaction mechanism (step by step picture of what bonds are broken and formed), one product is favored over the other. Why?

+Markovnikov’s Rule

This rule states that when an unsymmetrical molecule, such as HCl or HBr, is added to a C-C double bond, the hydrogen from the HX goes to the carbon atom that has the greater number of hydrogen atoms.

The reaction mechanism that goes along with this type of reaction generates what is known as a carbocation or a carbon with a temporary positive charge.

+Reaction Mechanism

+Carbocations

Carbocations are intermediate species shown in mechanisms that represent carbons with a positive charge.

There are 3 types of carbocations, depending on how many other carbons are attached to the one with the positive charge.

Primary (1°)- attached to one other carbon

Secondary (2°)- attached to 2 other carbons

Tertiary (3°)- attached to 3 other carbons

Cannot have any others!

The following order represents the most stable carbocations:

3°>2°>1°>CH3

+Practice

Write the products formed when 2-methyl-1-butene reacts with:

A) H2, Pt at 25°C

B) Cl2

C) HCl

D) H2O, H+

Write equations for the addition of HCl to:

A) 1-pentene

B) 2-pentene

+Stereoisomers

When branches are added as a result of alkene reactions, it is important to know if they are added to the same side of the molecule or opposite sides of the molecule.

If they are added to the same side of the molecule, we call them syn.

If they are added to the opposite sides of the molecule, we call them anti.

Let's see some examples!

+Oxidation Reaction for Alkenes

Alkenes can be oxidized by adding cold, dilute potassium permanganate (Baeyer test) to form a dihydroxy alcohol).

This test determines the presence of double or triple bonds in a molecule.

+Alkynes

Rules for naming:

Same as rules for alkenes, except main chain ends in –yne.

Practice: 4) 5)

1) 3-methyl-1-butyne

2) 4-ethyl-4-methyl-2-hexyne

3) cyclohexylethyne

+Preparation

These compounds are very reactive!

However, ethyne is relatively easy to make using calcium carbide and water, or by the cracking of methane in an electric arc.

+Physical and Chemical Properties of Alkynes

Physical:

All of these properties are of acetylene, which is the common name for ethyne. It’s the most common alkyne.

Insoluble in water.

Gas at normal temperature.

May decompose violently, spontaneously or from shock.

Chemical:

Acetylene is most often used as fuel for welding/cutting torches or an intermediate in manufacture of other substances.

+Alkyne Reactions

Addition reactions:

Burns hot! 2800 °C.

Undergoes addition reactions like ethane with Cl, Br, HCl, HBr.

Alkynes also follow Markovnikov’s rule for addition reactions.

Other reactions are possible that alkenes do not do:

Acetylene with HCN to form acrylonitrile.

+Aromatics

All structures that contain a benzene ring are considered aromatic.

Michael Faraday first isolated benzene in 1825.

It was a couple of years before the formula, C6H6, was determined.

In 1865, Kekule proposed a structure in which each carbon had 1 H attached and there were three double bonds alternating with single bonds.

However, with this model, there should be 2 dibromobenzenes, but there were not. SO…

+Aromatic structure

He suggested that the bonds must be in rapid oscillation. (Like the original idea behind resonance).

No one could really determine the correct structure behind benzene until 1912, when x-ray diffraction allowed chemists to measure the distance between carbon-carbon nuclei in bonds.

Since all of the bonds in benzene are in-between a single and double bond length, they must all share the same bond type.

+Naming Aromatics

These are benzene rings that have a group replacing one of the hydrogens on the ring.

Most of these are named by using the substituted group as a prefix in front of benzene.

Some have specific names:

Monosubstituted benznes

Toluene

Styrene

Phenol

+Disubstituted benzenes

When more than 2 groups replace hydrogens on a benzene ring, three isomers are possible:

1) Ortho

2) Meta

3) Para

With respect to A, as shown, there are two ortho positions… if the 1 carbon is A and the other branch is B, it would have to be in the 2 position either clockwise or counterclockwise. The meta would be 1,3, so there are 2 of those also.

There is only 1 para position.

+Naming disubstituted benzenes

Use ortho, meta or para and then the branch names…

Example: ortho dichlorobenzene

If the branches are not the same, name them alphabetically, but use ortho, para or meta to describetheir location to each other.

Example: meta bromomethylbenzene

• Dimethyl substituted benzene rings have a special name: xylene

+Polysubstituted Benzene Rings

When more than 2 substituents are on benzene, the carbon atoms are numbered starting at one of the substituent groups.

You must travel in the way that gives the groups the lowest numbers of carbons possible, either clockwise or counterclockwise.

If it is named as a derivative of a special parent compound (like toluene or phenol), the group of the parent compound is carbon #1.

Example:

2,4,6-trinitrotoluene (TNT)

+Practice

Write formulas and names for all the isomers of

A) chloronitrobenzene

(nitro is a NO2group)

B) tribromobenzene

C) chlorophenol

Name the following:

A)

B)

+Polycyclic Aromatic Compounds

Polycyclic or fused rings consist of structures where 2 carbons are common to two rings.

These fused rings exist in many of the amino acids and other natural biochemical substances.

Three of the most common in chemical world are napthalene, anthracene and phenanthrene.

Napthalene

Anthracene

Phenanthrene

+Carcinogens or helpful? BOTH

A number of benzene and polycyclic structures have been found to be carcinogenic, or cancer causing.

Some of the more notable ones are found in coal tar and tar from cigarette smoke.

Many aromatics were first derived from coal tar upon intense heating.

Since our source of coal tar is limited, we have looked to petroleum to make aromatics like benzene, toluene and xylene.

Used in the production of:

Dyes

Pharmaceuticals

Detergents

Insecticides

Plastics

Rubber

Explosive

Carcinogens Helpful

+Physical and Chemical Properties of Aromatics

Nonpolar

Less dense than water

Miscible in many organic solvents, but not water

Most are liquids or solids

Most undergo substitution of a group on the benzene ring for a hydrogen.

Typical substitutions:

1) Halogenation- benzene reacts with chlorine or bromine to become substituted

Physical properties Chemical properties

+More chemical properties/reactions

2) Nitration- benzene reacts with concentrated nitric and sulfuric acid at 50 Celsius to form nitrobenzene.

3) Alkylation- (also called Friedel –Crafts reaction) the alkyl group from an alkyl halide, in the presence of AlCl3 as a catalyst, adds the halogen to the benzene ring

+Mechanism of substitution

Electrophilic substitution-

3 steps:

1) An electrophile (electron seeking group) is formed.

2) Electrophile is attached to the benzene ring, forming a positively charged carbocation intermediate.

3) A hydrogen ion is lost from the carbocation to form the product.

+Side Chain Oxidation

Carbon chains attached to aromatic rings are easy to oxidize.

Most common reactants used to oxidize are KMnO4 or K2Cr2O7 with sulfuric acid (H2SO4).

No matter how long the carbon chain is of the group attached to the ring, it will form a carboxylic acid.

Carboxylic acid- COOH group