Chapter 25 Hydrocarbon Compounds

Section 25.1Hydrocarbons

OBJECTIVES:

–Describe the bonding in hydrocarbons.

Section 25.1Hydrocarbons

OBJECTIVES:

–Distinguish between straight-chain and branched-chain alkanes.

Organic Chemistry and Hydrocarbons

Organic originally meant chemicals that came from organisms

1828 German chemist Friedrich Wohler synthesized urea in a lab

Today, organic chemistry is the chemistry of virtually all compounds containing the element carbon

Organic Chemistry and Hydrocarbons

Over a million organic compounds, with a dazzling array of properties

Why so many? Carbon’s unique bonding ability!

Let’s start with the simplest of the organic compounds: Hydrocarbons

Organic Chemistry and Hydrocarbons

Hydrocarbons contain only two elements: hydrogen and carbon– simplest hydrocarbons called

alkanes, which contain only single covalent bonds

– methane (CH4) with one carbon is the simplest alkane. It is the major component of natural gas

Organic Chemistry and Hydrocarbons

Review structural formula- p.744 Carbon has 4 valence electrons,

thus forms 4 covalent bonds– not only with other elements, but also

forms bonds WITH ITSELF.

Ethane (C2H6) is the simplest alkane with a carbon to carbon bond

Straight-Chain Alkanes

Straight-chain alkanes contain any number of carbon atoms, one after the other, in a chain -meaning one linked to the nextC-C-C C-C-C-C etc.

Names of alkanes always will always end with -ane

Straight-Chain Alkanes

Combined with the -ane ending is a prefix for the number of carbons– Table 25.1, page 745

Homologous series- a group of compounds that have a constant increment of change

In alkanes, it is: -CH2-

Straight-Chain Alkanes Many alkanes used for fuels:

methane, propane, butane, octane As the number of carbons increases,

so does the boiling and melting pt.– The first 4 are gases; #5-15 are liquids;

higher alkanes are solids Condensed structural formulas?

Note examples on page 746

Naming Straight-Chain Alkanes

Names recommended by IUPAC - the International Union of Pure and Applied Chemistry– end with -ane, the root part of the name

indicates the # of carbons We sometimes still rely on common

names, some of which are well-known

Naming Straight-Chain Alkanes

IUPAC names may be long and cumbersome

Common names may be easier or more familiar, but usually do not describe the chemical structure!– Methane is natural gas or swamp gas

Branched-Chain Alkanes

Branched-chain means that other elements besides hydrogen may be attached to the carbon– halogens, oxygen, nitrogen, sulfur, etc.

– any atom that takes the place of a hydrogen on a parent hydrocarbon is called a substituent, or the branched part

Branched-Chain Alkanes A hydrocarbon substituent is called

an alkyl group or sometimes radicals– use the same prefixes to indicate the

number of carbons, but the -ane ending is now -yl methyl, ethyl, propyl, etc.

Gives much more variety to the organic compounds

Branched-Chain Alkanes Rules for naming - page 748

1. Longest chain is parent

2. Number so branches have low #

3. Give position number to branch

4. Prefix more than one branch

5. Alphabetize branches

6. Use proper punctuation ( - , )

Branched-Chain Alkanes Sample 25-2, page 749 From the name, draw the structure:

1. Find the parent, with the -ane

2. Number carbons on parent

3. Identify substituent groups; attach

4. Add remaining hydrogens

Sample 25-3, page 750

Alkanes 3-ethylpentane 2,3,4-trimethylhexane Since the electrons are shared

equally, the molecule is nonpolar– thus, not attracted to water

– oil (a hydrocarbon) not soluble in H2O

– “like dissolves like”

Section 25.2Unsaturated Hydrocarbons

OBJECTIVES:

–Explain the difference between unsaturated and saturated hydrocarbons.

Section 25.2Unsaturated Hydrocarbons

OBJECTIVES:

–Differentiate between the structures of alkenes and alkynes.

Alkenes

Multiple bonds can also exist between the carbon atoms

Hydrocarbons containing carbon to carbon double bonds are called alkenes C=C C-C=C

Called “unsaturated” if they contain double or triple bonds

Naming Alkenes Find longest parent that has the

double bond in it New ending: -ene Number the chain, so that the double

bond gets the lower number Name and number the substituents Samples on page 752

Alkynes Hydrocarbons containing carbon to

carbon triple bonds called alkynes

-C C- Alkynes are not plentiful in nature Simplest is ethyne- common name

acetylene (fuel for torches) Table 25.2, page 753 for b.p. and m.p.

Section 25.3Isomerism

OBJECTIVES:

–Distinguish among structural, geometric, and stereoisomers.

Section 25.3Isomerism

OBJECTIVES:

–Identify the asymmetric carbon or carbons in stereoisomers.

Structural Isomers

Compounds that have the same molecular formula, but different molecular structures, are called structural isomers

Butane and 2-methylpropane Also have different properties, such

as b.p., m.p., and reactivity

Geometric Isomers There is a lack of rotation around a

carbon to carbon multiple bond– has an important structural implication

– Two possible methyl arrangements:

1. trans configuration - substituted groups on opposite sides of double bond

2. cis configuration - same side

Geometric Isomers

Trans-2-butene and Cis-2-butene shown on page 754

differ only in the geometry of their substituted groups

like other structural isomers, have different physical and chemical properties (also note page 755-top)

Stereoisomers Don’t forget that these structures are

really 3-dimensional stereoisomers- molecules of the same

molecular structure that differ only in the arrangement of the atoms in space- bottom page 755

Asymmetric carbon? C with 4 different groups attached (Sample 25-4, p.756)

Section 25.4Hydrocarbon Rings

OBJECTIVES:

–Identify common cyclic ring structures.

Section 25.4Hydrocarbon Rings

OBJECTIVES:

–Explain resonance in terms of the aromatic ring of benzene.

Cyclic Hydrocarbons The two ends of the carbon chain

are attached in a ring in a cyclic hydrocarbon– sample drawings on page 759– named as “cyclo- ____”

hydrocarbon compounds that do NOT contain rings are known as aliphatic compounds

Aromatic Hydrocarbons A special group of unsaturated cyclic

hydrocarbons is known as arenes– contain single rings, or groups of rings

– originally called “aromatic hydrocarbons”, because of pleasant odor

– simplest arene is benzene (C6H6)

– Term “aromatic” applies to materials with bonding like that of benzene

Aromatic Hydrocarbons

Benzene is a six-carbon ring, with alternating double and single bonds– exhibits resonance, due to location of

the double and single bonds-p.760 Benzene derivatives possible:

– methylbenzene, 3-phenylhexane, ethylbenzene- page 760

Aromatic Hydrocarbons

Benzene derivatives can have two or more substitutents:– 1,2-dimethylbenzene

– 1,3-dimethylbenzene

– 1,4-dimethylbenzene Can use ortho for 1,2; meta for 1,3;

and para for 1,4 (page 761)

Section 25.5Hydrocarbons from the Earth

OBJECTIVES:

–Identify three important fossil fuels and describe their origins.

Section 25.5Hydrocarbons from the Earth

OBJECTIVES:

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

Natural Gas

Fossil fuels provide much of the world’s energy

Natural gas and petroleum contain mostly aliphatic (straight-chain) hydrocarbons

Natural gas is an important source of alkanes of low molecular mass

Natural Gas

Natural gas is typically:– 80% methane, 10% ethane, 4%

propane, and 2% butane with the remainder being nitrogen and higher molar mass hydrocarbons

– also contains a small amount of He, and is one of it’s major sources

Natural Gas Natural gas is prized for combustion,

because with adequate oxygen, it burns with a hot, clean blue flame:– CH4 + 2O2 CO2 + 2H2O + heat

Insufficient burning has a yellow flame, due to glowing carbon parts, as well as making carbon monoxide

Petroleum The compounds found in petroleum

(or crude oil) are more complex than those in natural gas

Usually straight-chain and branched-chain alkanes, with some aromatic compounds also

Crude oil must be refined (separated) before being used

Petroleum It is separated by distillation into

fractions, according to boiling pt. Fractions containing higher molar

mass can be “cracked” into more useful shorter chain components, such as gasoline and kerosene– involves catalyst and heat

– starts materials for plastics and paints

Coal From huge fern trees and mosses

decaying millions of years ago under great pressure

Stages in coal formation:

1. Peat- soft, fibrous material much like decayed garden refuse; high water content. After drying will make a low-cost, smoky fuel

Coal2. Lignite- peat left in the ground

longer, loses it’s fibrous texture, and is also called brown coal– harder than peat; higher C content

(50%); still has high water content

3. Bituminous, or soft coal- formed after more time; lower water content, higher C content (70-80%)

Coal4. Anthracite, or hard coal

– carbon content exceeding 80%, making it an excellent fuel source

Coal may be found close to the surface (strip-mined), or deep within the earth

Pollutants from coal are common; soot and sulfur problems

Coal Coal may be distilled for many

products– coal gas, coal tar, coke, and ammonia

– further distilled into benzene, toluene, naphthalene, phenol, and pitch

– Coke is almost pure carbon; produces intense heat and little or no smoke, thus used in industrial processes