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 next 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 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 branchedpart
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
Coal 2. 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%)
Coal 4. 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