Presentation on theme: "Chapter 25 Hydrocarbon Compounds. Section 25.1 Hydrocarbons l OBJECTIVES: –Describe the bonding in hydrocarbons."— Presentation transcript:
Chapter 25 Hydrocarbon Compounds
Section 25.1 Hydrocarbons l OBJECTIVES: –Describe the bonding in hydrocarbons.
Section 25.1 Hydrocarbons l OBJECTIVES: –Distinguish between straight- chain and branched-chain alkanes.
Organic Chemistry and Hydrocarbons l Organic originally meant chemicals that came from organisms l 1828 German chemist Friedrich Wohler synthesized urea in a lab l Today, organic chemistry is the chemistry of virtually all compounds containing the element carbon
Organic Chemistry and Hydrocarbons l Over a million organic compounds, with a dazzling array of properties l Why so many? Carbon’s unique bonding ability! l Let’s start with the simplest of the organic compounds: Hydrocarbons
Organic Chemistry and Hydrocarbons l Hydrocarbons contain only two elements: hydrogen and carbon –simplest hydrocarbons called alkanes, which contain only single covalent bonds –methane (CH 4 ) with one carbon is the simplest alkane. It is the major component of natural gas
Organic Chemistry and Hydrocarbons l Review structural formula- p.744 l Carbon has 4 valence electrons, thus forms 4 covalent bonds –not only with other elements, but also forms bonds WITH ITSELF. l Ethane (C 2 H 6 ) is the simplest alkane with a carbon to carbon bond
Straight-Chain Alkanes l 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. l Names of alkanes always will always end with -ane
Straight-Chain Alkanes l Combined with the -ane ending is a prefix for the number of carbons –Table 25.1, page 745 l Homologous series- a group of compounds that have a constant increment of change l In alkanes, it is: -CH 2 -
Straight-Chain Alkanes l Many alkanes used for fuels: methane, propane, butane, octane l 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 l Condensed structural formulas? Note examples on page 746
Naming Straight-Chain Alkanes l 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 l We sometimes still rely on common names, some of which are well-known
Naming Straight-Chain Alkanes l IUPAC names may be long and cumbersome l 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 l 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 l 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. l Gives much more variety to the organic compounds
Branched-Chain Alkanes l Rules for naming - page 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 l Sample 25-2, page 749 l 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 l 3-ethylpentane l 2,3,4-trimethylhexane l Since the electrons are shared equally, the molecule is nonpolar –thus, not attracted to water –oil (a hydrocarbon) not soluble in H 2 O –“like dissolves like”
Section 25.2 Unsaturated Hydrocarbons l OBJECTIVES: –Explain the difference between unsaturated and saturated hydrocarbons.
Section 25.2 Unsaturated Hydrocarbons l OBJECTIVES: –Differentiate between the structures of alkenes and alkynes.
Alkenes l Multiple bonds can also exist between the carbon atoms l Hydrocarbons containing carbon to carbon double bonds are called alkenes C=C C-C=C l Called “unsaturated” if they contain double or triple bonds
Naming Alkenes l Find longest parent that has the double bond in it l New ending: -ene l Number the chain, so that the double bond gets the lower number l Name and number the substituents l Samples on page 752
Alkynes l Hydrocarbons containing carbon to carbon triple bonds called alkynes -C C- l Alkynes are not plentiful in nature l Simplest is ethyne- common name acetylene (fuel for torches) l Table 25.2, page 753 for b.p. and m.p.
Section 25.3 Isomerism l OBJECTIVES: –Distinguish among structural, geometric, and stereoisomers.
Section 25.3 Isomerism l OBJECTIVES: –Identify the asymmetric carbon or carbons in stereoisomers.
Structural Isomers l Compounds that have the same molecular formula, but different molecular structures, are called structural isomers l Butane and 2-methylpropane l Also have different properties, such as b.p., m.p., and reactivity
Geometric Isomers l 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 l Trans-2-butene and Cis-2-butene shown on page 754 l differ only in the geometry of their substituted groups l like other structural isomers, have different physical and chemical properties (also note page 755-top)
Stereoisomers l Don’t forget that these structures are really 3-dimensional l stereoisomers- molecules of the same molecular structure that differ only in the arrangement of the atoms in space- bottom page 755 l Asymmetric carbon? C with 4 different groups attached (Sample 25-4, p.756)
Section 25.4 Hydrocarbon Rings l OBJECTIVES: –Identify common cyclic ring structures.
Section 25.4 Hydrocarbon Rings l OBJECTIVES: –Explain resonance in terms of the aromatic ring of benzene.
Cyclic Hydrocarbons l The two ends of the carbon chain are attached in a ring in a cyclic hydrocarbon –sample drawings on page 759 –named as “cyclo- ____” l hydrocarbon compounds that do NOT contain rings are known as aliphatic compounds
Aromatic Hydrocarbons l 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 (C 6 H 6 ) –Term “aromatic” applies to materials with bonding like that of benzene
Aromatic Hydrocarbons l 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 l Benzene derivatives possible: –methylbenzene, 3-phenylhexane, ethylbenzene- page 760
Aromatic Hydrocarbons l Benzene derivatives can have two or more substitutents: –1,2-dimethylbenzene –1,3-dimethylbenzene –1,4-dimethylbenzene l Can use ortho for 1,2; meta for 1,3; and para for 1,4 (page 761)
Section 25.5 Hydrocarbons from the Earth l OBJECTIVES: –Identify three important fossil fuels and describe their origins.
Section 25.5 Hydrocarbons from the Earth l OBJECTIVES: –Name some products obtained from natural gas, petroleum, and coal.
Natural Gas l Fossil fuels provide much of the world’s energy l Natural gas and petroleum contain mostly aliphatic (straight-chain) hydrocarbons l Natural gas is an important source of alkanes of low molecular mass
Natural Gas l 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 l Natural gas is prized for combustion, because with adequate oxygen, it burns with a hot, clean blue flame: –CH 4 + 2O 2 CO 2 + 2H 2 O + heat l Insufficient burning has a yellow flame, due to glowing carbon parts, as well as making carbon monoxide
Petroleum l The compounds found in petroleum (or crude oil) are more complex than those in natural gas l Usually straight-chain and branched-chain alkanes, with some aromatic compounds also l Crude oil must be refined (separated) before being used
Petroleum l It is separated by distillation into fractions, according to boiling pt. l 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 l From huge fern trees and mosses decaying millions of years ago under great pressure l 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 l Coal may be found close to the surface (strip-mined), or deep within the earth l Pollutants from coal are common; soot and sulfur problems
Coal l 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