chapter 25 “hydrocarbon compounds” chemistry golden valley high school stephen l. cotton
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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
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