Chapter 25 “Hydrocarbon Compounds”

ChemistryGolden Valley High School

Stephen L. Cotton

Section 25.1Hydrocarbons

OBJECTIVES:

–Describe the relationship between number of valence electrons and bonding in carbon.

Section 25.1Hydrocarbons

OBJECTIVES:

–Define and describe alkanes.

Section 25.1Hydrocarbons

OBJECTIVES:

–Relate the polarity of hydrocarbons to their solubility.

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

Friedrich Wohler

1800 – 1882

Used inorganic substances to synthesize urea, a carbon compound found in urine.

This re-defined organic chemistry.

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: 1) hydrogen, & 2) carbon– simplest hydrocarbons called “alkanes”,

which contain only carbon to carbon single covalent bonds (CnH2n+2)

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

Organic Chemistry and Hydrocarbons

Carbon has 4 valence electrons, thus forms 4 covalent bonds– not only with other elements, but also

forms bonds WITH ITSELF (nonpolar)

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

Straight-Chain AlkanesStraight-chain alkanes contain

any number of carbon atoms, one after the other, in a chain -meaning one linked to the next (not always straight)C-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 carbonsTable 25.1, page 745

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

In alkanes, it is: -CH2- (methylene)

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

Naming Straight-Chain Alkanes

Names recommended by IUPAC - the International Union of Pure and Applied Chemistryend 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, and even other carbons–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 -ane ending is now -yl such as: methyl, ethyl, propyl, etc.

Gives much more variety to the organic compounds

Branched-Chain AlkanesRules for naming – go from right

to left - page 7481. Longest C-C chain is parent2. Number so branches have lowest #3. Give position number to branch4. Prefix (di, tri) more than one branch5. Alphabetize branches (not prefix)6. Use proper punctuation ( - and , )

- Page 699

Branched-Chain Alkanes

From the name, draw the structure, in a right-to-left manner:

1. Find the parent, with the -ane

2. Number carbons on parent

3. Identify substituent groups (give lowest number); attach

4. Add remaining hydrogens

- Page 700

Properties of Alkanes Draw 3-ethylpentane Draw 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”

Quiz

Draw. 2,3 diethyl pentane 3,3, 4 tri-methyl octane 2,3 ethyl, methyl hexane

Name

C C

C-C-C-C-C-C

Section 25.2Unsaturated Hydrocarbons

OBJECTIVES:

–Describe the difference between unsaturated and saturated hydrocarbons.

Section 25.2Unsaturated Hydrocarbons

OBJECTIVES:

–Distinguish the structures of alkenes and alkynes.

AlkenesMultiple bonds can also exist

between the carbon atomsHydrocarbons containing

carbon to carbon double bonds are called alkenes (CnH2n) 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 702

AlkynesHydrocarbons containing carbon

to carbon triple bonds called alkynes

(CnH2n-2) -C C-Alkynes are not plentiful in natureSimplest is ethyne- common

name acetylene (fuel for torches)Table 22.3, p. 703 for boiling pt.

Section 25.3Isomers

OBJECTIVES:

–Explain why structural isomers have different properties.

Section 25.3Isomers

OBJECTIVES:

–Describe the conditions under which geometric isomers are possible.

Section 25.3Isomers

OBJECTIVES:

–Identify optical isomers.

Structural Isomers Compounds that have the same

molecular formula, but different molecular structures, are called structural isomers

Butane and 2-methylpropane (make by breaking Carbon off the end, and making a branch in the middle)

Also have different properties, such as b.p., m.p., and reactivity

Structural Isomers of Butane, C4H10

StereoisomersDon’t forget that these structures

are really 3-dimensionalstereoisomers- molecules of the

same molecular structure that differ only in the arrangement of the atoms in space. Two types are a) geometric and b) optical

Geometric Isomers There is a lack of rotation around a

carbon to carbon multiple bond– has an important structural implication

– Two possible arrangements:

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

2. cis configuration - same side

Substituted groups are on opposite sides of the double bond (in this case, one is above, the other is below)

Substituted groups are on the same side of the double bond (in this case, both are above)

Geometric Isomers

Trans-2-butene

Cis-2-butene

Geometric Isomers

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

differ in the geometry of the substituted groups (to double bond)

like other structural isomers, have different physical and chemical properties

Optical IsomersAsymmetric carbon? C with 4

different groups attached. Conceptual Problem 25.12, p.755

Molecules containing asymmetric carbons have “handedness”, and exist as stereoisomers.

Optical Isomers, and these will each show an assymetric carbon (4 different branches attached)

The assymetric carbon

Section 25.4Hydrocarbon Rings

OBJECTIVES:

–Identify cyclic ring structures.

Section 25.4Hydrocarbon Rings

OBJECTIVES:

–Describe bonding in 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

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

– simplest aromatic 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 761

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)

C

C

CC

Section 25.5Hydrocarbons From Earth’s Crust

OBJECTIVES:

–Identify three important fossil fuels and describe their origins.

Section 25.5Hydrocarbons From Earth’s Crust

OBJECTIVES:

–Describe the composition of natural gas, petroleum, and coal.

Section 25.5Hydrocarbons From Earth’s Crust

OBJECTIVES:

–Describe what happens when petroleum is refined.

Natural Gas Fossil fuels provide much of the

world’s energy Natural gas and petroleum contain

mostly the aliphatic (or straight-chain) hydrocarbons – formed from marine life buried in sediment of the oceans

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

Incomplete 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 of rocks / soil.

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

BIG BRUTUS

Dragline used to remove the

overburden of a strip mining coal field near West Mineral, Kansas

Note the man standing beside it

Coal Coal may be distilled for many products

–coal gas, coal tar, coke, and ammonia

–further distilled into benzene, toluene, naphthalene, phenol- the aromatics

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