Chapter Eleven Unsaturated Hydrocarbons. Copyright © Houghton Mifflin Company. All rights reserved.11–2 Alkenes, Alkynes, and Aromatics Alkanes are often

Chapter Eleven

Unsaturated Hydrocarbons

Copyright © Houghton Mifflin Company. All rights reserved. 11–2

Alkenes, Alkynes, and Aromatics

• Alkanes are often referred to as saturated because each carbon atom bonds to the maximum number of hydrogen atoms and no more hydrogen atoms can be added.

• Alkenes and alkynes are referred to as unsaturated because they contain carbon-carbon double and triple bonds to which more hydrogen atoms can be added.


Aromatic hydrocarbons exhibit a special type of “delocalized” bonding that usually involves a six-membered carbon ring (Section 11.8).

A functional group is a specific part of a molecule, a cluster of atoms with special properties. A carbon-carbon multiple bond is a functional group in an unsaturated hydrocarbon.

In an unsaturated hydrocarbon reactions can occur at the multiple bonds: an example would be the addition of hydrogen atoms at the bond positions.


Characteristics of Alkenes, Cycloalkenes, and Alkynes


• Alkenes are hydrocarbons that contain carbon-carbon double bonds.

• Alkynes are hydrocarbons that contain carbon-carbon triple bonds.


Three-dimensional representations of the structures of ethene and methane. In ethene, the atoms are in a flat (planar) form rather than a tetrahedral arrangement. The bond angles in ethene are 120º.

Source: James and Kara Birk

Note: The common name of etheneis ethylene.


The general formula for an alkene is CnH2n and the general formula for an alkyne is CnH2n-2.

CH3-CH2-CH2-CH=CH-CH3

C6H12

C6H10

CH3-CH2-CH2-C C-CH3


Cycloalkenes are cyclic unsaturated hydrocarbons that contain one or more carbon-carbon double bonds within the ring system.

cyclohexene

A general formula for cycloalkenes containing only one double bond is CnH2n-2.


Naming Alkenes and Alkynes

• IUPAC nomenclature rules for alkenes and alkynes are similar to alkanes. • Step 1. Name the parent compound. Find the longest chain containing the double or triple bond, and name the parent compound by adding the suffix –ene or –yne to the name of the main chain.


• Step 2: Number the carbon atoms in the main chain, beginning at the end nearer to the double or triple bond. If the multiple bond is an equal distance from both ends, begin numbering at the end nearer the first branch point.

• Step 3: Write the full name. Assign numbers to the branching substituents, and list the substituents alphabetically. Use commas to separate numbers, and hyphens to separate words from numbers. Indicate the position ofthe double bond carbon.


C-C-C-C-C-C-C=C-C-C

Step 1: The longest straight chain = 10 carbons, i.e., decane. Since it is an alkene drop the ane suffix and add ene = decene.

Step 2: Number to give the functional group (the double bond) the lowest number. The correct nameIs 3-decene.

EXAMPLE


If more than one multiple bond is present, identify the position of each multiple bond and use the appropriate ending diene, triene, tetraene, and so forth.

1 3 1

4

1,3-cyclohexadiene 1,4-cyclohexadiene


Note: The functional group must be present in theparent chain.

C-C-C-C-C-C-C-C-C=C-C

2-undecene

C-C-C-C-C-C-C-C-C-C

C=C

How would you name the following molecule?


The Structures of Alkenes

• Recall that methane is tetrahedral, ethylene is planar, and acetylene is linear.


CIS-TRANS ISOMERS

Unlike carbon-carbon single bonds, rotation around a carbon-carbon double bond is very difficult. As a result, a new kind of isomerism is possible for alkenes. For example, there are two different kinds of 2-butenes. These are called cis-trans isomers.

C C C C==C C==CC

cis-2-butene trans-2-butene


• In the cis isomer, the two methyl groups are close together on the same side of the double bond.


• In the trans isomer, the two methyl groups are far apart on opposite sides of the double bond.

• The cis and trans isomers have the same formula and the same connections between the atoms, but they have different three-dimensional structures because of the way the groups are attached to the carbons.


• Cis-trans isomerism occurs in an alkene whenever the double-bond carbons are bonded to two different substituent groups. Cis-trans isomerism is not present when one of the double-bond carbons is attached to two identical groups.


Cis - trans isomers: Different representations of the cis and trans

isomers of 2-butene.

Source: James and Kara Birk


Comparison of Structural Isomers for Four- and Five-Carbon Alkane and Alkene Systems.


Physical and Chemical Properties of Alkenes


Properties of Alkenes and Alkynes

• Nonpolar, insoluble in water, soluble in nonpolar organic solvents (recall “like dissolves like”).

• Less dense than water: they float on water.

• Flammable

• Generally nontoxic (minor problems may occur)

• Alkenes display cis-trans isomerism whereas alkynes do not.

• Both alkenes and alkynes are chemically reactive—the multiple bonds are reaction centers


Kinds of Organic Reactions

• Addition reaction: A substance adds to the multiple bond of an unsaturated reactant to yield a saturated product that has only single bonds.

• Elimination reaction: In which a saturated reactant yields an unsaturated product by losing groups from two adjacent carbons.


• Symmetrical Addition Reactions: In which identical groups are added to each carbon of the double bond. For example, hydrogenation or halogenation addition reactions.

• Unsymmetrical Addition Reactions: In which different groups are added to each carbon of the double bond. For example, hydration or hydrohalogenation.


Reactions of Alkenes and Alkynes

• Addition of H2 to alkenes and alkynes: Alkenes and alkynes react with hydrogen in the presence of a metal catalyst such as palladium to yield the corresponding saturated alkane products. This is called hydrogenation. It is a symmetrical addition reaction.


• Addition of Cl2 and Br2 to alkenes: halogenation. Alkenes react with the halogens Br2 and Cl2 to give the 1,2-dihaloalkanes. This is also a symmetrical addition reaction.


A solution of bromine in water is reddish brown (left). When a small amount of unsaturated hydrocarbon is added to such a solution, the resulting solution is decolorized because the bromine adds to the hydrocarbon to form colorless dibromo compounds (right).


• Addition of HCl and HBr to alkenes:

Alkenes react with hydrogen bromide and hydrogen chloride to give alkyl bromide or alkyl chloride products. This is an unsymmetrical addition reaction.


Markovnikov rule: In the addition of HX to an alkene, the H becomes attached to the carbon that already has more H’s, and X becomes attached to the carbon that has fewer H’s.


In an alkene addition reaction, the atoms provided by an incoming molecule are attached to the carbon atoms originally joined by a double bond. In the process, the double bond becomes a single bond.


Addition of water to alkenes: Hydration.

An alcohol is produced on treatment of an alkene with water in the presence of a strong acid catalyst (such as H2SO4). (Markovnikov’s rule can be used to predict the product when water adds to an unsymmetrically substituted alkene.)


Let’s try this: Let H2O attack propene:

H2O

CH3CH==CH2 ???

Where does the H go? Is the product …

CH3CH2CH2OH (normal propyl

alcohol)

or CH3CH-CH3 ? (isopropyl

alcohol)

OH


How Does an Alkene Addition Reaction

Occur?

Reaction mechanism = A description of the individual steps by which old bonds are broken and new bonds are formed.

Consider the following two-step mechanism for addition of HBr to an alkene.


In the 1st step, the alkene reacts with H+ from the HBr and produces a carbocation (positive ion with the + charge on carbon).

In the 2nd step, this reactive carbocation quickly reacts with Br- ion to form the product.

CH3-CH2-CH2-CH=CH2HBr

CH3-CH2-CH2-CH-CH3

Br

Br

Carbocations have a positive charge on a carbon atom. Secondary carbocations are formed in preference toprimary carbocations.

The H attaches here


Polymers of Alkenes

Polymer: A large molecule formed by the repetitive bonding together of many smaller molecules called monomers. Many simple alkenes undergo polymerization reactions when treated with the proper catalyst.

H2C=CH

Zpolymerization

catalystC C

H

H

C

H

H Z

H

C

H

H

n

polyethylene

excess Z = H


An addition polymer is a polymer in which the monomerssimply “add together” with no other products formedbesides the polymer.

An example of an addition polymer is polystyrene.

HC CH2* *

nn

polymerize

“n” molecules of styrene polymerize to form polystyrene.

styrene = vinyl benzene

Note: “vinyl” refers to an ethylene (H2C=CH2) substituent


When styrene is heated with a catalyst (benzoyl peroxide), it yields a viscous liquid. After some time, this liquid sets to a hard plastic (sample shown at left).

Source: James Scherer


Addition Polymers


A co-polymer is one in which two different monomersare used and a polymer is formed which contains bothmonomers in the polymeric chain. An important co-polymer is styrene-butadiene rubber. It containsthe monomers styrene and 1,3-butadiene in a 1:3 ratio.This polymer is a major ingredient in automobile tires.

Saran wrap is a co-polymer of vinyl chloride (chloroethene) and 1,1-dichloroethene.

C C

Cl

* CCl

H H

H Cl

Cl H

H

+

Cl

Cl

C

H H

H

*

n

H

H

polymerize

Saran Wrap


Some Common Polymers Obtained from Ethene - Based Monomers.


Chemistry at a Glance:Chemical Reactions of Alkenes


Alkynes


An alkyne is an acyclic unsaturated hydrocarbon in which one or more carbon-carbon triple bonds are present.

CH3-CH2-CH2-C CH

1-pentyne

4-decyne


Structural representations of ethyne (acetylene), the simplest alkyne.


Chemical Reactions of Alkynes

CH3HCH3

H

H

HH2

catalyst

Alkynes are reduced to alkenes in the presence ofhydrogen and a catalyst.

Alkynes are insoluble in water, but soluble in organicsolvents.


Aromatic Hydrocarbons


An historical note


Aromatic Compounds and the Structure of Benzene

• In the early days the word aromatic was used to described many fragrant molecules isolated from natural sources. Today the term aromatic is used to describe benzene-like molecules.

• Benzene is a flat, symmetrical molecule with the molecular formula C6H6.

• It can be drawn with alternating carbon-carbon double and single bonds.


• Unlike alkenes, benzene does not undergo addition reactions.

• Benzene’s relative lack of chemical reactivity is due to its structure.

• There are two possible structures with alternating double and single bonds. These are called “resonance structures.”


• Experimental evidence shows that all six carbon-carbon bonds in benzene are identical (with the same bond length).

• The properties, including the above one, of benzene can only be explained by assuming that the actual structure of benzene is an average of the two equivalent structures— this is referred to as resonance.

• Simple aromatic compounds like benzene are non-polar, insoluble in water, volatile, and flammable.

• Unlike alkenes, several aromatic hydrocarbons are toxic. Benzene itself is implicated as a cancer-causing chemical.


Names for Aromatic Hydrocarbons


Naming Aromatic Compounds

• Substituted benzenes are named using benzene as the parent. No number is needed for mono-substituted benzene since all the ring positions are identical.

• Disubstituted aromatics are named using one of the prefixes ortho, meta-, or para-.

• An ortho- or o-disubstituted aromatic has its two substituents in the 1,2-relationship on the ring.


• A meta- or m-disubstituted aromatic has its two substituents in the 1,3-relationship on the ring.

• A para- or p-disubstituted aromatic has its two substituents in the 1,4-relationship on the ring.

X X X

X

X

X

ortho meta para

Para-dichlorobenzene is used as modern moth balls.



When more than two groups are present on the benzene ring, their positions are indicated with numbers. The ring is numbered so as to obtain the lowest possible numbers for the carbon atoms that have substituents.

The group that comes first alphabetically is given the lowest number.

Note: di-, tri-, and tetra- do not count in determiningalphabetical order. Isopropyl is considered an “i” and not a “p”.


Many substituted aromatic compounds have common names in addition to their systematic names.


Fused-ring aromatic hydrocarbons are aromaticcompounds with two or more rings fused together.

Benz[a]pyrene is a carcinogen (cancer-causing chemical) found in cigarette smoke and automobile exhaust; naphthalene was formerly used as moth balls.

benzene naphthalene anthracene benz[a]pyrene


Chemical Portraits:Single and Double-Ring Aromatic Hydrocarbons


Properties and Reactions of Aromatic Hydrocarbons


Reactions of Aromatic Compounds

Unlike alkenes which undergo addition reactions, aromatic compounds undergo substitution reactions. That is, a group Y substitutes for one of the hydrogen atoms on the aromatic ring. An example is given below:

YH


Some Common Aromatic Reactions:

Nitration: Substitution of a nitro group (-NO2) for a ring hydrogen.

Halogenation: Substitution of a halogen (Cl or Br) for a ring hydrogen.

Sulfonation: Substitution of a sulfonic (-SO3H) group for a ring hydrogen.

Alkylation: Substitution of an alkyl group (-CH2CH3, etc) for a ring hydrogen.


NO2H

HNO3/H2SO4

BrH

Br2/FeBr3

SO3HH

SO3/H2SO4

CH2CH3H

CH3CH2Cl

AlCl3

Nitration

Halogenation

Sulfonation

Alkylation


Key Things to Understandin Chapter 11

--The term “unsaturated” means that multiple bonds are present in a compound.

--Alkenes contain carbon-carbon double bonds.--Alkynes contain carbon-carbon triple bonds.--Aromatic compounds contain six carbons in a

ring arrangement with three double and three single bonds alternating between carbon atoms. In reality, all the bonds are equivalent due to “resonance.”

--Alkenes are named using the family ending –ene, while the alkynes use the family ending –yne.

--Alkenes and alkynes generally undergo addition reactions and aromatic compounds generally undergo substitution reactions.


--A “reaction mechanism” is a description of the individual steps by which old bonds are broken and new bonds are formed.

--In unsaturated compounds the multiple bonds are common targets for reaction.

--Ethene (a.k.a., ethylene, H2C==CH2) is a planar (flat) molecule with 120˚ bond angles.

--Ethyne (a.k.a. acetylene, H-C≡C-H) is a linear (straight-chain) molecule with 180˚ bond angles.

--(Recall that methane, CH4, has H-C-H bond angles of 109.5˚, the “tetrahedral angle.”)

--The general formula for an alkene is CnH2n and the general formula for an alkyne is CnH2n-2.

--Cycloalkenes are ring compounds containing one or more C=C double bonds.

Key Things (cont’d)


--Cycloalkynes are ring compounds containing one or more C≡C bonds.

--Know how to name alkenes, alkynes, cycloalkenes, and cycloalkynes.

--Understand, and be able to identify, cis and trans isomers of alkenes.

--Understand that alkenes and alkynes are nonpolar, don’t dissolve in water, float on water, and are reactive (e.g., they are flammable).

--Understand that “addition reactions” involve the addition of compounds (e.g., H2 or HBr) across the double bond. They can be symmetrical (H2) or unsymmetrical (HBr).

--Be able to use the Markovnikov rule to predict the product of an addition reaction.



--Understand that “polymers” are large molecules formed by linking small molecules called “monomers.”

--Understand that addtion polymers are formed by linking monomers by means of addition reactions (e.g., polyethylene is formed from ethylene monomers).

--Be prepared to identify “styrene” (benzene with an ethene group stuck on it) and polystyrene.

--Understand that a copolymer involves linking two different types of monomers into a polymer.

--Understand that “hydrogenation” reactions reduce the bond order of a multiple bond.

--Understand that benzene is the prototype “aromatic” compound.



--Appreciate that although we may sometimes draw benzene as having alternating double and single C-C bonds, all six C-C bonds are in fact equivalent, halfway between a double and a single C-C bond.

--Be prepared to name di-substituted benzenes using the ortho-, meta-, para- description.

--Understand that several benzene derivatives (phenol, aniline, toluene) are very important and have their own special names.

--Be able to recognize the two most common fused-ring aromatic compounds, naphthylene (two rings) and anthracene (three rings).

--Understand that aromatic compounds typically undergo “substitution” reactions, in which a new functional group replaces an H atom.



To Do List

• Read chapter 11!!

• Do additional problems

• Review Lecture notes forChapter Eleven

• Do practice test T/F

• Do practice test MC

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Chapter Eleven Unsaturated Hydrocarbons. Copyright © Houghton Mifflin Company. All rights reserved.11–2 Alkenes, Alkynes, and Aromatics Alkanes are often