Chapter 25/26: Simple Organic Chemistry

Basic Structure and Nomenclature

Graphic: www.lab-initio.com

Chapter 25 Organic Chemistry Goals• 25.1 Saturated Hydrocarbons

– Describe bonding in hydrocarbons– Distinguish straight chain vs. branched chain

alkanes• 25.2 Unsaturated hydrocarbons

– Explain saturated vs. unsaturated hydrocarbons– Differentiate alkenes, alkynes

• 25.3 Isomerism– Distinguish among structural, geometric, and

stereoisomers.– Identify the asymmetric carbon or carbons in

stereoisomers

Chapter 25 Organic Chemistry Goals

• 25.4 Hydrocarbon Rings– Identify common cyclic ring structures– Explain resonance in terms of the aromatic ring

of benzene• 25.5 Hydrocarbons from the Earth

– Identify 3 important fossil fuels and their origins

– Name some products obtained from natural gas, petroleum and coal.

25.1 Hydrocarbons• Hydrocarbons are organic compounds composed of

Carbon and Hydrogen only.• Alkanes – hydrocarbons with all single bonds (no

double or triple bonds)• Methane

– CH4 this is the main component in natural gas.

– Rotting things and cows produce and emit methane.

• Note that carbon always has four covalent bonds because it has four valence electrons.

• Carbon bonds using a tetrahedral form to spread out H’s as much as possible due to repulsion.

H

H

H

H

H

H

H

H C

28.1 Alkanes• Ethane is the next one in the sequence, with two

carbons and six hydrogens, where both carbons are tetrahedrons. It is also a gas at STP, like methane is.

• Above are four different ways of representing methane. – 1st: Lewis dot– 2nd: 2D - Carbons at the junctions is implied– 3rd: Ball and stick model– 4th: Space fill model

H

H

H H

H

HC C

H H

H

H

H

H

Straight Chain Alkanes

• C3H8 propane

• C4H10 butane

• Straight chain alkanes contain any number of carbon atoms, one after the other in a chain.

• As you can see, they aren’t always straight though.

C C

H H

H

H H

H

H

H

H

HH

H

HC

H

H

H

C C

H H

H

H H

C

H

H

H

H

H

H

H

H

H

H

H

HC

H

H

H

First Ten AlkanesFormula

Name Formula

Name

CH4 Methane C6H14 Hexane

C2H6 Ethane C7H16 Heptane

C3H8 Propane C8H18 Octane

C4H10 Butane C9H20 Nonane

C5H12 Pentane C10H22 Decane

• Alkane = CnH2n+2

• Note they all end in –ane• It’s a homologous series with

increment –CH2

AlkanesName Molecu

le Formula

Structural Formula Boiling point (oC)

Methane

CH4 CH4 -161.0

Ethane C2H6 CH3CH3 -88.5

Propane C3H8 CH3CH2CH3 -42.0

Butane C4H10 CH3CH2CH2CH3 0.5

Pentane C5H12 CH3CH2CH2CH2CH3 36.0

Hexane C6H14 CH3CH2CH2CH2CH2CH3 68.7

Heptane

C7H16 CH3CH2CH2CH2CH2CH2CH3 98.5

Octane C8H18 CH3CH2CH2CH2CH2CH2CH2CH3 125.6

Nonane C9H20 CH3CH2CH2CH2CH2CH2CH2CH2CH3 150.7

Decane C10H22 CH3CH2CH2CH2CH2CH2CH2CH2CH2CH

3

174.1What is the trend with boiling point? Why?

Straight Chain Alkanes aren’t “Straight”

CH3

CH3

C – C bonds are sp3 hybridized (tetrahedral)

Butane, C4H10

Structural Shorthand

H

HH

H

HH

H

H

HH

Explicit hydrogens (those required to complete carbon’s valence) are usually left off of drawings of hydrocarbons

CH3

CH3

Line intersections represent carbon atoms

C1 C1C2 C2

C3 C3C4 C4

Branched Chain Alkanes

• A hydrocarbon substituent is called an alkyl group.

• On the drawing on the left, that is a methyl group on the top. You start by using the name methane, and removing the –ane and adding –yl instead.

• When a substituent alkyl group is added to a straight-chain hydrocarbon, branches are formed. An alkane with one or more alkyl groups is called a branched chain alkane.

• Let’s look at the rules for naming these (next page):

Rules for Naming Alkanes (Nomenclature)

For a branched hydrocarbon, the longest continuous chain

of carbon atoms gives the root name for the hydrocarbon

CH3

CH3

CH3

12 3

4

4 carbon chain = butane

Rules for Naming Alkanes (Nomenclature)

When alkane groups appear as substituents, they

are named by dropping the -ane and adding -yl.

CH3

CH3

CH3

—CH3 Methyl

—CH2CH3 Ethyl

—CH2CH2CH3 Propyl—CH2CH2CH2CH3 Butyl

Methyl

Rules for Naming Alkanes (Nomenclature)

The positions of substituent groups are specified

by numbering the longest chain of carbon atoms

sequentially, starting at the end closest to the

branching.CH3

CH3

CH3

Methyl

12 3

4

Rules for Naming Alkanes (Nomenclature)

The location and name of each substituent are followed by the root alkane name. The substituents are listed in alphabetical order (irrespective of any prefix – for example, ethyl comes before methyl), and the prefixes di-, tri-, etc. are used to indicate multiple identical substituents.

CH3

CH3

CH3

Methyl

12 3

4 Name:2-methylbutane

Nomenclature Practice

CH3 CH3

CH3

CH3

Cl

Name this compound

Step #1: For a branched hydrocarbon, the longest continuous chain of carbon atoms gives the root name for the hydrocarbon

152 43

9

6

87

9 carbons = nonane

Nomenclature PracticeName this compound

CH3 CH3

CH3

CH3

Cl

152 43

9

6

87

9 carbons = nonane

Step #2: When alkane groups appear as substituents, they are named by dropping the -ane and adding -yl.

CH3 = methyl

chlorine = chloro

Nomenclature PracticeName this compound

CH3 CH3

CH3

CH3

Cl

152 43

9

6

87

9 carbons = nonane

CH3 = methyl

chlorine = chloro

Step #3: The positions of substituent groups are specified by numbering the longest chain of carbon atoms sequentially, starting at the end closest to the branching.

1 9 NOT 9 1

Nomenclature PracticeName this compound

CH3 CH3

CH3

CH3

Cl

152 43

9

6

87

9 carbons = nonane

CH3 = methyl

chlorine = chloro

Step #4: The location and name of each substituent are followed by the root alkane name. The substituents are listed in alphabetical order (irrespective of any prefix, meaning chloro comes before methyl), and the prefixes di-, tri-, etc. are used to indicate multiple identical substituents.

2-chloro-3,6-dimethylnonane

Branched Chain Alkanes Rules Summary

1. Find the longest chain of carbons in the molecule (doesn’t have to be straight or linear). Here it’s 7, so the parent hydrocarbon is heptane.

2. Number the carbons in the main chain in sequence as shown. To do this, you must start at the end that will end up giving the alkyl groups the smallest number for the carbon they are attached to. Here they are on carbon # 2,3,4. If you numbered it the other way they would have been on 4,5,6.

Branched Chain Naming (contin.)

3. Add numbers to the names of the substituent groups to identify their positions on the chain. These numbers become prefixes that come before the name of the parent alkane. Here we have : 2-methyl, 3-methyl, 4-ethyl.

4. Use prefixes to indicate the appearance of a group more than once. Use di- (twice), tri- (three times), tetra- (four times) and penta- (five times). In the structure above, we’ll be using dimethyl

Branched Chain Naming (contin.)

5. List the names of the alkyl substituents in alphabetical order, ignoring the di-, tri- part. For example, ethyl groups will come before methyl groups.

6. Commas are used to separate numbers, hyphens are used to separate numbers and words. There are no spaces anywhere in the name!

7. So the compound above is: 4-ethyl-2,3-dimethylheptane

Sample Problem 25-2

• Name the above compounds using the rules given on the last slides.

a. The chain is six carbons long (longest configuration). There are two methyl groups on the 3rd carbon. The name is 3,3-dimethylhexaneb. The chain is five carbons long. There are two methyl groups on 2nd and 4th carbon. The name is 2,2,4,4-tetramethylpentane

Sample problem 25-3

• Draw structural formulas for the following:

• 3-ethylhexane

• 2,2,4-trimethylpentane

Properties of Alkanes• The electron pairs in hydrogen-carbon bonds

are shared nearly equally between the nuclei of the C and H. Therefore hydrocarbon compounds such as alkanes are nonpolar.

• Therefore the alkanes of low molar mass tend to be gases or low boiling point liquids (refer to slide #8 for boiling points).

• Since water is a polar molecule, nonpolar organic materials such as alkanes are not attracted to water. You already know that oil and water do not mix because they don’t meet the rule that “like dissolves like” because one is polar (water) and the other is nonpolar (alkanes/oil/hydrocarbons).

Structural Isomers

CH3

CH3

CH3

CH3 CH3

CH3

CH3

Isomers are molecules with the same chemical formula, but different organization of atoms (different bonding). Here are some examples using branched and unbranched alkanes. They are all examples of C5H12.

n-Pentane, C5H12

Isopentane, C5H12

Neopentane, C5H12

CH3 CH3

Cyclic AlkanesCyclopropane, C3H6

Remember, explicit hydrogens are left out

Cyclobutane, C4H8

Cyclopentane, C5H10

Cyclohexane, C6H12

Cycloheptane, C7H14

25.2 Unsaturated hydrocarbons

• An unsaturated hydrocarbon compound is similar to an alkane but it has double or triple bonds.

• For every double bond, there is one less H atom on the carbon. Molecules with double bonds are alkenes.

• For every triple bond, there are two less H atoms on the carbon. Molecules with triple bonds are alkynes.

• The word “saturated” means there are the maximum number of H atoms possible, meaning there are only single bonds on the molecule.

• The word “unsaturated” means there are less than the maximum number of H atoms possible, which means there are double or triple bonds somewhere.

Alkenes• An alkene, ethene left and below left• Other alkenes below right• Note the “2-pentene” and “2-butene”:

you also have to specify where the double bonds are!

Alkenes• When working with an alkene, you will find the

longest chain in the molecule that contains the double bond, and that will be the parent alkene.

• It has the same root name as the alkane with the same number of carbons, but the ending changes to –ene.

• The smallest alkenes are ethene and propene, which are often called by the common names ethylene and propylene.

• Did you notice on that last page that ethene is planar?

• The CH2 groups on the ends of the double bond can’t be rotated about the double bond, so no rotation occurs about a C=C double bond.

Alkynes

• Alkynes have triple CΞC bonds.

• This is ethyne.• Note that the

triple bond is rigid, so no rotation about this bond can occur.

Boiling PointsWhat do you notice about the trend in boiling points as a function of single, double and triple bonds?Anything?

25.3 Isomers• Some structures of hydrocarbons are similar,

differing only in the way the groups are assembled.

• Here’s an example, two ways to build C4H10 :

• Structural isomers like these two differ in physical properties such as boiling point, melting point, and chemical reactivity.

• Generally the more branched, the lower boiling point.

Geometric Isomers• Because a double C=C bond cannot rotate, that has

some important structural implications.• There are two possible arrangements for the methyl

groups on 2-butene, the cis and the trans configuration.

• The cis configuration has both groups on the same side of the double bond.

• The trans has the groups on opposite sides of C=C.

trans cis

Cis and Trans Geometric Isomers

Identical chemical formula and bonding, just different geometric position.

cis trans

Geometric Isomers (continued)

• Cis-1,2-dichloroethene Trans-1,2-dichloroethene

The cis and trans isomers can have physical properties that are different, especially if there is a polar atom like Cl on there.

Stereoisomers • Molecules that we draw on paper in two

dimensions can look the same on paper but different in three dimensions if they are nonsuperimposable mirror images of each other.

• Your two hands are an example of what a steroisomer is like. They are nonsuperimposable mirror images of each other, and they cannot be placed on top of each other to obtain a match.

• Do you see how these two appear to be mirror images of each other?• With four different atoms attached like this, it is an assymmetric carbon.

Sample Problem 25-4

Here you are asked to evaluate if the compounds have an asymmetric carbon

25.4 Hydrocarbon Rings

• Cyclic hydrocarbons– Compounds that contain a hydrocarbon ring

exist when the two ends of the carbon chain are attached to each other.

– Hydrocarbon compounds that do NOT contain rings (Sections 25.1-25.3) are called aliphatic compounds.

– Five- and six-carbon rings are the most abundant.

– Shapes like these are used to represent the ring, with a carbon at each junction.

25.4 Hydrocarbon Rings, saturated

Hydrocarbon Rings, Aromatic

• A special group of unsaturated cyclic hydrocarbons are known as arenes.

• These can be rings or groups of rings. • They were originally called aromatic compounds,

meaning that they had sweet, pleasant odors.• The simplest aromatic hydrocarbon is benzene,

C6H6

This is one of four (or more) ways to show benzene. The single and double bonds alternate positions, so they show dotted lines to represent that. See next slide for more…

Benzene ring – aromatic hydrocarbon

H

H

H

H

H

H

• Benzene is a six-carbon ring (C6H6) with one H attached to each C. That leaves one electron free from each Carbon to participate in a double bond.

• Benzene is a cyclic unsaturated hydrocarbon with delocalized electrons, which results in resonance structures.

H

H

H

H

H

H

OR…H

H

H

H

H

H

See how the drawing on the left and center are resonance structures that are represented by the drawing on the right?

More ways to show benzene…

• This shows a benzene ring group (called a phenyl group) attached to hexane on the 3 carbon. When a benzene is a substituent on an alkane, it is called a phenyl group.

• This one is an ethyl group attached to a benzene ring as a substituent. This compound is called ethylbenzene.

Geometric Isomerism in Aromatics

Cl

Cl

H

H

H

H

Cl

Cl

H

H

H

H

ortho (o-) = two adjacent substituents

meta (m-) = one carbon between substituents

para (p-) = two carbons between substituents

p-dichlorobenzene

o-dichlorobenzene

m-dichlorobenzene

Cl

HH

H

Cl

H

25.5 Hydrocarbons from the Earth• Natural gas

– Natural gas is an important source of alkanes of low molar mass.

– Natural gas is typically about 80% methane, 10% ethane, 4% propane and 2% butane.

– Natural gas is good for combustion (your gas stove, your water heater) because it burns with a nice hot clean flame to form CO2 and water.

– Propane and butane, which are separated from the other gases by liquifaction, are also good heating fuels. They are sold in liquid forms.• I have a 500 gallon propane tank in my front

yard because there is no natural gas where I live.

Petroleum• Petroleum typically has the higher molar mass

hydrocarbons that compose it. Cracking is the process where large molecules are broken into smaller ones.

• Most are straight chain and branched alkanes.• Petroleum does have a small amount of aromatic

compounds and sulfur, oxygen and nitrogen containing compounds.

• Crude oil gets distilled to divide it into fractions according to its boiling point (fractionation into individual chemicals).

• The crude oil is heated so that it vaporizes and rises through a column. Compounds with the highest boiling points condense first near the bottom and compounds with the lowest boiling points at the top.

Fractionation column to separate crude oil into usable components.

Fractions obtained from Crude Oil

Fraction Composition of carbon chains

Boiling range in oC

Percent of crude oil

Natural Gas C1 to C4 Below 20Below 10%

Petroleum ether (solvent)

C5 to C6 30 to 60

Naphtha (solvent) C7 to C8 60 to 90

Gasoline C6 to C12 40 to 175 40%

Kerosene C12 to C15 150 to 275 10%

Fuel oils, mineral oil C12 to C15 225 to 400 30%

Lubricating jelly, petroleum jelly, greases, paraffin wax, asphalt

C16 to C24 Over 400 10%

Coal

• Coal has its origin about 300 million years ago when forests and swamps were buried under layers of vegetation and decayed under intense pressure.

• The pressure, coupled with the heat from the Earth’s interior, turned the plant remains into coal.

• In some places, such as Eastern Pennsylvania, the hardest and most carbon rich coal, anthracite, was produced. Anthracite has a carbon content exceeding 80% which makes it an excellent fuel source.

• Coal is usually found underground in seams 1-3 meters thick. In America, most coal is less than 100 meters underground. In Europe, it may be as much as 1500 meters underground.

Coal• Coal is mostly condensed ring compounds with

extremely high molar mass. • If you recall benzene, it’s ratio of C to H was 1 to

1, unlike hexane which is 6 carbons to 14. Because of this, coal leaves more soot upon burning than do the more aliphatic (non-ring) compounds.

• Another issue with coal is that it can have as much as 7% sulfur, which creates SO2 and SO3 air pollution.

• Coal can be used to produce (through distillation) products such as coal gas (H2 + CH4 + CO), coal tar, coke (fuel for industrial processes) and ammonia (to make fertilizer).

• Coal tar can be used to produce benzene, toluene, naphthalene, phenol and pitch.

26.1 Introduction to Functional Groups

• A functional group is a specific arrangement of atoms in an organic compound that is capable of characteristic chemical reactions.

• Organic compounds can be classified according to their functional groups.

• The double and triple bonds of alkene and alkyne groups are chemically reactive so they are considered functional groups as well.

Hydrocarbon Functional Groups

Class Functional Group General Formula

Alcohol hydroxyl group -O — H R – OH

Alkyl halide or halocarbon

halogen — X R — X

Ether — O — R — O — R’

Aldehyde carbonyl group O with H || — C — H

O || R — C — H

Ketone carbonyl group O || — C —

O || R — C — R’

Carboxylic acid carboxyl group O || — C —

OH

O || R — C — OH

Ester O || — C — O—

O || R — C — O — R’

Amine (also see amide in table 26.1)

amine group | —N—

R’ | R — N — R’’

Halogen Substituents• Halocarbons are organic compounds with a halogen

bonded onto them (F, Cl, Br, I). • Halocarbons in which a halogen attached to a

carbon of an aliphatic chain (non-ring) are called alkyl halides.

• Halocarbons in which a halogen attaches to an aromatic hydrocarbon (or arene) are called aryl halides.

Space filling models of the three above >>

Why do you think the boiling point changes so much as a function of how many chlorines are on a methane?

Recall that hydrocarbons are nonpolar typically. The addition of Cl makes them polar. The more polar they are, the more the attraction they have to overcome to boil off and escape in the gas phase.

Halocarbons

• Halocarbons are easily prepared from hydrocarbons.

• Hydrofluorocarbons (freons) are used as refrigerants.

• A common way to prepare halocarbons is using a substitution reaction. A halogen can replace a hydrogen on an alkane. The symbol X stands for the halogen in this reaction below:

R-H + X2 → R-X + HX

Alkane Halogen Halocarbon Hydrogen halide• Here’s an example:• CH4 + Cl2 → CH3Cl + HCl

Halocarbons (continued)• Halogens on carbon chains can readily be displaced

to produce an alcohol and a salt. (Purpose is to make an alcohol).

• The general reaction is as follows:• R-X + OH-1 → R-OH + X- Hydrocarbon hydroxide ion alcohol halide ion (salt)

• Here’s an example: H2O, 100oC

CH3I(l) + KOH (aq) → CH3OH (l) + KI (aq)