2Section 25.1 Hydrocarbons OBJECTIVES: Describe the bonding in hydrocarbons.
3Section 25.1 Hydrocarbons OBJECTIVES: Distinguish between straight-chain and branched-chain alkanes.
4Organic Chemistry and Hydrocarbons Organic originally meant chemicals that came from organisms1828 German chemist Friedrich Wohler synthesized urea in a labToday, organic chemistry is the chemistry of virtually all compounds containing the element carbon
5Organic Chemistry and Hydrocarbons Over a million organic compounds, with a dazzling array of propertiesWhy so many? Carbon’s unique bonding ability!Let’s start with the simplest of the organic compounds: Hydrocarbons
6Organic Chemistry and Hydrocarbons Hydrocarbons contain only two elements: hydrogen and carbonsimplest hydrocarbons called alkanes, which contain only single covalent bondsmethane (CH4) with one carbon is the simplest alkane. It is the major component of natural gas
7Organic Chemistry and Hydrocarbons Review structural formula- p.744Carbon has 4 valence electrons, thus forms 4 covalent bondsnot only with other elements, but also forms bonds WITH ITSELF.Ethane (C2H6) is the simplest alkane with a carbon to carbon bond
8Straight-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
9Straight-Chain Alkanes Combined with the -ane ending is a prefix for the number of carbonsTable 25.1, page 745Homologous series- a group of compounds that have a constant increment of changeIn alkanes, it is: -CH2-
10Straight-Chain Alkanes Many alkanes used for fuels: methane, propane, butane, octaneAs the number of carbons increases, so does the boiling and melting pt.The first 4 are gases; #5-15 are liquids; higher alkanes are solidsCondensed structural formulas? Note examples on page 746
11Naming 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 carbonsWe sometimes still rely on common names, some of which are well-known
12Naming Straight-Chain Alkanes IUPAC names may be long and cumbersomeCommon names may be easier or more familiar, but usually do not describe the chemical structure!Methane is natural gas or swamp gas
14Branched-Chain Alkanes Branched-chain means that other elements besides hydrogen may be attached to the carbonhalogens, 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
15Branched-Chain Alkanes A hydrocarbon substituent is called an alkyl group or sometimes radicalsuse 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
16Branched-Chain Alkanes Rules for naming - page 7481. Longest chain is parent2. Number so branches have low #3. Give position number to branch4. Prefix more than one branch5. Alphabetize branches6. Use proper punctuation ( - , )
17Branched-Chain Alkanes Sample 25-2, page 749From the name, draw the structure:1. Find the parent, with the -ane2. Number carbons on parent3. Identify substituent groups; attach4. Add remaining hydrogensSample 25-3, page 750
18Alkanes 3-ethylpentane 2,3,4-trimethylhexane Since the electrons are shared equally, the molecule is nonpolarthus, not attracted to wateroil (a hydrocarbon) not soluble in H2O“like dissolves like”
19Section 25.2 Unsaturated Hydrocarbons OBJECTIVES:Explain the difference between unsaturated and saturated hydrocarbons.
20Section 25.2 Unsaturated Hydrocarbons OBJECTIVES:Differentiate between the structures of alkenes and alkynes.
21Alkenes Multiple bonds can also exist between the carbon atoms Hydrocarbons containing carbon to carbon double bonds are called alkenes C=C C-C=CCalled “unsaturated” if they contain double or triple bonds
22Naming Alkenes Find longest parent that has the double bond in it New ending: -eneNumber the chain, so that the double bond gets the lower numberName and number the substituentsSamples on page 752
23AlkynesHydrocarbons containing carbon to carbon triple bonds called alkynes-C C-Alkynes are not plentiful in natureSimplest is ethyne- common name acetylene (fuel for torches)Table 25.2, page 753 for b.p. and m.p.
24Section 25.3 Isomerism OBJECTIVES: Distinguish among structural, geometric, and stereoisomers.
25Section 25.3 Isomerism OBJECTIVES: Identify the asymmetric carbon or carbons in stereoisomers.
26Structural IsomersCompounds that have the same molecular formula, but different molecular structures, are called structural isomersButane and 2-methylpropaneAlso have different properties, such as b.p., m.p., and reactivity
27Geometric IsomersThere is a lack of rotation around a carbon to carbon multiple bondhas an important structural implicationTwo possible methyl arrangements:1. trans configuration - substituted groups on opposite sides of double bond2. cis configuration - same side
28Geometric Isomers Trans-2-butene and Cis-2-butene shown on page 754 differ only in the geometry of their substituted groupslike other structural isomers, have different physical and chemical properties (also note page 755-top)
29StereoisomersDon’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- bottom page 755Asymmetric carbon? C with 4 different groups attached (Sample 25-4, p.756)
30Section 25.4 Hydrocarbon Rings OBJECTIVES:Identify common cyclic ring structures.
31Section 25.4 Hydrocarbon Rings OBJECTIVES:Explain resonance in terms of the aromatic ring of benzene.
32Cyclic HydrocarbonsThe two ends of the carbon chain are attached in a ring in a cyclic hydrocarbonsample drawings on page 759named as “cyclo- ____”hydrocarbon compounds that do NOT contain rings are known as aliphatic compounds
33Aromatic Hydrocarbons A special group of unsaturated cyclic hydrocarbons is known as arenescontain single rings, or groups of ringsoriginally called “aromatic hydrocarbons”, because of pleasant odorsimplest arene is benzene (C6H6)Term “aromatic” applies to materials with bonding like that of benzene
34Aromatic Hydrocarbons Benzene is a six-carbon ring, with alternating double and single bondsexhibits resonance, due to location of the double and single bonds-p.760Benzene derivatives possible:methylbenzene, 3-phenylhexane, ethylbenzene- page 760
35Aromatic Hydrocarbons Benzene derivatives can have two or more substitutents:1,2-dimethylbenzene1,3-dimethylbenzene1,4-dimethylbenzeneCan use ortho for 1,2; meta for 1,3; and para for 1,4 (page 761)
36Section 25.5 Hydrocarbons from the Earth OBJECTIVES:Identify three important fossil fuels and describe their origins.
37Section 25.5 Hydrocarbons from the Earth OBJECTIVES:Name some products obtained from natural gas, petroleum, and coal.
38Natural Gas Fossil fuels provide much of the world’s energy Natural gas and petroleum contain mostly aliphatic (straight-chain) hydrocarbonsNatural gas is an important source of alkanes of low molecular mass
39Natural Gas Natural gas is typically: 80% methane, 10% ethane, 4% propane, and 2% butane with the remainder being nitrogen and higher molar mass hydrocarbonsalso contains a small amount of He, and is one of it’s major sources
40Natural GasNatural gas is prized for combustion, because with adequate oxygen, it burns with a hot, clean blue flame:CH4 + 2O2 CO2 + 2H2O + heatInsufficient burning has a yellow flame, due to glowing carbon parts, as well as making carbon monoxide
41PetroleumThe compounds found in petroleum (or crude oil) are more complex than those in natural gasUsually straight-chain and branched-chain alkanes, with some aromatic compounds alsoCrude oil must be refined (separated) before being used
42PetroleumIt 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 keroseneinvolves catalyst and heatstarts materials for plastics and paints
43CoalFrom huge fern trees and mosses decaying millions of years ago under great pressureStages 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
44Coal2. Lignite- peat left in the ground longer, loses it’s fibrous texture, and is also called brown coalharder than peat; higher C content (50%); still has high water content3. Bituminous, or soft coal- formed after more time; lower water content, higher C content (70-80%)
45Coal 4. Anthracite, or hard coal carbon content exceeding 80%, making it an excellent fuel sourceCoal may be found close to the surface (strip-mined), or deep within the earthPollutants from coal are common; soot and sulfur problems
46Coal Coal may be distilled for many products coal gas, coal tar, coke, and ammoniafurther distilled into benzene, toluene, naphthalene, phenol, and pitchCoke is almost pure carbon; produces intense heat and little or no smoke, thus used in industrial processes