Organic Chemistry Chapter 20

 Carbon is only the 17 th most abundant element in the Earth’s crust, but it is vitally important because it is found in every living thing and in fuels that we use (gas, oil, wood).

Structure and Bonding of Carbon  Carbon has 4 valence electrons, 2s and 2p.  It actually forms what’s called an sp 3 hybrid, in a tetrahedral shape, and tends to form 4 bonds.  Carbon can form double bonds through sp 2 hybridization and triple bonds through sp hybridization.

Allotropes of Carbon o Diamond: hardest material known to man  Density is 3.5 x that of water  Melting point above 3500°C  Each carbon is tetrahedrally bonded to three other carbons  Conducts heat 5 x more readily than silver or copper (the best metallic conductors). The heat is transferred by vibrational motion from one carbon to another (works well because they are close to each other and the bonds are strong enough to transfer vibratory motion)  Does NOT conduct electricity because electrons are not free-moving

o Graphite: soft and greasy-feeling  Carbons are in layers of thin hexagonal plates  The layers are too far apart to be held together by covalent bonds- only weak London dispersion forces hold them. So layers can slide across one another— graphite can be used as a lubricant and for pencil lead.  Carbons are bonded to only three other carbons, so there are delocalized electrons: electrical conductor!  Melting point = 3652°C.

o Fullerenes: newly discovered in 1980’s  C-60 is the most stable, with alternating hexagons and pentagons  Amazing properties! can also make nanotubes!  Tensile strength > 37 GPa (steel = 2 GPa)  Young’s modulus (bending/twisting) ~ TPa (steel =.3 TPa)  Density ~1.4 g/cm 3 (steel ~8 g/cm 3 and aluminum = 2.7 g/cm 3 )  Electrical resistivity ~1μΩcm (copper = 1.7 μΩcm)  Thermal conductivity ~ 3000 W/mK (diamond ~2000 W/mK)

Organic Compounds  Defined as covalently bonded compounds containing carbon, excluding carbonates and oxides (Na 2 CO 3, CO, CO 2 )  Catenation: the covalent bonding of an element to itself to form chains or rings  Hydrocarbons: composed of only carbon and hydrogen (simplest organic cmpds)

 Isomers: have the same molecular formula but different structures—organic cmpds are often shown with structural formulas in order to show the bonding arrangement of atoms  Isomers differ in both physical and chemical properties!  The structural formula does not accurately show the three-dimensional shape of the molecule.

 Structural isomers: isomers in which the atoms are bonded together in different orders. Example: C 4 H 10.

 Structural isomers can have different physical or chemical properties.  Example: butane and methylpropane have different melting points, boiling points, and densities.

 Geometric isomers: isomers in which the order of atom bonding is the same but the arrangement of atoms in space is different.  In order for these to exist, there must be a rigid structure in the molecule to prevent free rotation around a bond (or then the atoms could rotate and would be identical).

 Because the two chlorine atoms are on the same side of the molecule in the first structure, it is called cis.  In the second molecule, the two chlorine atoms are on opposite sides of the molecule, and so the molecule is called trans.  In both molecules, the bonding order of the atoms is the same: each carbon atom in the double bond is also bound to one chlorine atom and one hydrogen atom.

 Natural unsaturated fatty acids are cis-fatty acids.  When vegetable oil, which contains unsaturated fatty acids, is converted by hydrogenation into a solid fat (like margarine or vegetable shortening), trans-fatty acids are produced.  Diets high in trans-fatty acids may cause health risks!  Some pheromones, which are geometric isomers of each other, are physiologically active in insects and the isomer is only slightly active or not at all!

 These at first appear to be different, but they are actually the same—in each one, there are two hydrogen atoms on one side and one hydrogen and on chlorine on the other.  A molecule can have a geometric isomer only if two carbons in a rigid structure have two different groups attached.

Alkanes  Hydrocarbons that contain only single bonds are alkanes.

Saturated Hydrocarbons  Hydrocarbons are grouped mainly by the type of bonding between carbon atoms.  Hydrocarbons in which each carbon atom in the molecule forms four single covalent bonds with other atoms—all bonds are “full” or “saturated.

 Homologous series is one in which adjacent members differ by a constant unit. C n H 2n+2  Example: C 2 H 6 propane.  Ethane: n = 2, so there are two carbon atoms and (2 x 2) + 2 = 6 hydrogen atoms.  Suppose a member has 30 carbons. What would the formula look like?

Naming Alkanes  The name depends on the longest carbon chain, and those are based on prefixes set by IUPAC (Int’l Union of Pure and Applied Chemistry): Number of Cprefix 1meth- 2eth- 3prop- 4but- 5pent- 6hex- 7hept- 8oct- 9non- 10dec-

CH 3 —CH 2 —CH 2 —CH 2 —CH 2 —CH 2 —CH 3 H H H H H—C—C—C—C—H H H H H

Branched-Chain Alkane Nomenclature  Alkyl groups are groups of atoms that are formed when one hydrogen atom is removed from an alkane molecule.  Straight chain with 7 carbons in it would be heptane.  Once the chain becomes branched, you must name the longest part of the chain using the prefix method, and then name each branch as an alkyl group

AlkaneNameAlkyl GroupName CH4Methane--CH3Methyl CH3—CH3Ethane--CH2—CH3Ethyl CH3—CH2—CH3Propane--CH2—CH2—CH3Propyl CH3—CH2—CH2—CH3Butane--CH2—CH2—CH2—CH3Butyl CH3—CH2—CH2—CH2—CH3Pentane--CH2—CH2—CH2—CH2—CH3Pentyl

 Alkyl groups are groups of atoms that are formed when one hydrogen atom is removed from an alkane molecule.  Alkyl groups are named by replacing the suffix –ane of the parent alkane with the suffix –yl.

CH 3 CH 3 CH 3 —CH 2 —CH 2 —CH—CH—CH—CH 2 —CH 3 CH—CH 3 CH 3  To name this molecule, locate the parent hydrocarbon.

 This molecule contains two chains that are eight carbon atoms long.  The parent hydrocarbon is the chain that contains the most straight-chain branches.

 To name the parent hydrocarbon, add the suffix –ane to the prefix oct- to form ______________?  The three –CH3 groups are methyl groups.  The –CH2—CH3 groups is an ethyl group.  Arrange the names in alphabetical order in front of the name of the parent hydrocarbon. ______________?

 To show that there are three methyl groups present, attach the prefix tri- to the name methyl to form trimethyl. ____________________?

 Now we need to show the locations of the alkyl groups on the parent hydrocarbon.  Number the octane chain so that the alkyl groups have the lowest possible numbers.  Place the location numbers of each of the alkyl groups in front of its name.  Separate the numbers from the names of the alkyl groups with hyphens.  The ethyl group is on carbon 3.  _____________________________?

 Because there are three methyl groups, there will be three numbers, separated by commas, in front of trimethyl.  The full name is: ___________________________________?

Summary of rules for naming alkanes: 1. Name the parent hydrocarbon : find the longest continuous chain of carbon atoms with straight-chain branches. Add –ane to the prefix matching the number of carbons. 2. Add the names of the alkyl groups, in the front of the alkane, in alphabetical order. If there is more than one of one kind of alkyl, add prefixes to say how many there are. Do this after they have been put in alpha order.

3. Number the carbon atoms in the parent hydrocarbon. Do this so that the branches have the lowest numbers possible. If there are two possibilities, give the lowest number to the alkyl group that comes first in the name. 4. Insert position numbers 5. Punctuate the name. separate position numbers from the names with hyphens, and if there is more than one number, use a comma.

Cycloalkanes  Alkanes in which the carbon atoms are arranged in a ring or cyclic structure.  Since there is no ‘free’ carbon at the end (it is bonded to another carbon atom), there are two fewer hydrogens, so the general formula is C n H 2n

Cycloalkane Nomenclature:  The cycloalkane is the parent.  Add the prefix cyclo- to the name of the straight-chain alkane.  When only one alkyl group is attached, there is no position number.  When there are more alkyl groups attached, the carbons are numbered so that the alkyl groups have the lowest numbers possible. So one alkyl group should always be in position 1!

 The structural formulas for cycloalkanes are often drawn in a simplified form.  It is understood that there is a carbon atom at each corner and enough hydrogen atoms to complete the four bonds to each carbon atom.

Properties of Alkanes:  Alkanes are nonpolar, so they have only weak intermolecular forces  The lowest molecular mass alkanes are gases: methane, propane, butane  Larger alkanes are liquids: gasoline and kerosene (C5-C10)  The largest alkanes are waxy solids, like paraffin  Boiling points increase regularly with number of Cs

Combustion of Alkanes :  Burn completely to form CO 2 and H 2 O  When fuels in an engine ignite before they are sparked, it causes engine knocking.  Straight-chain hydrocarbons are more likely to ignite spontaneously than branched hydrocarbons, so they cause more knocking.

 The more branched the fuel, the less knocking.  This is the basis for the octane rating of fuels!  isooctane, the common name of 2,2,4- trimethylpentane, is very resistant to knocking and has an octane rating of 100.  pure heptane has an octane rating of 0.

Unsaturated Hydrocarbons  Hydrocarbons in which not all carbon atoms have four single covalent bonds. That means that double and triple bonds can exist.  Alkenes : hydrocarbons with double bonds. The simplest alkene is ethene, with two carbons. The formula for an alkene with just one double bond is C n H 2n.  Because alkenes have double bonds, they can have geometric isomers.

 Rules are similar to those for alkanes; the name depends on the length of the chain containing the double bond.  If there is only one double bond, the suffix –ene is added to the carbon-chain prefix.  If there is more than one double bond, then the root name is modified: pentadiene, pentatriene, etc.

 Here, the longest chain that contains the double bond has five carbons and one double bond.  So the parent double bond is pentene. CH 2 —CH 3 CH 2 ==C--CH 2 --CH 2 --CH 3

 The carbon atoms in the chain are numbered so that the first carbon atom in the double bond has the lowest number.  The number indicating the position of the double bond is placed before the name of the hydrocarbon chain and separated by a hyphen.

 The position number and name of the alkyl group are placed in front of the double-bond position number.

 If there is more than one double bond, the suffix is modified to indicate the number of double bonds: 2 = -adiene, 3= -atriene CH 2 ==CH—CH 2 —CH==CH 2

 If numbering from both ends gives equivalent positions for the double bonds in an alkene with two double bonds, then the chain is numbered from the end nearest the first alkyl group. CH 3 CH 2 ==C—CH==CH 2

Alkynes  Alkynes are hydrocarbons with triple covalent bonds.The general formula for the alkynes is C n H 2n-2  Alkyne nomenclature is almost the same as alkene nomenclature.

 The difference is that the –ene suffix of the corresponding alkene is replaced with – yne.

1. Name the parent hydrocarbon.  Locate the longest continuous chain that contains the triple bond(s).  If there is only one triple bond, add the suffix – yne to the prefix corresponding to the number of carbon atoms in the chain.  If there is more than one triple bond, modify the suffix to indicate the number of triple bonds. For example, 2 = -adiyne 3 = -atriyne, and so on.

2. Add the names of the alkyl groups. 3. Number the carbon atoms in the parent hydrocarbon.  Number the carbon atoms in the chain so that the first carbon atom in the triple bond nearest the end of the chain has the lowest number.  If numbering from both ends gives the same positions for two triple bonds, then number from the end nearest the first alkyl group.

Aromatic Hydrocarbons  Aromatic hydrocarbons are hydrocarbons with six-membered carbon rings and delocalized electrons.  Benzene is the primary aromatic hydrocarbon.  The molecular formula of benzene is C 6 H 6.  One possible structural formula is a six-carbon atom ring with three double bonds.

 Benzene does not behave chemically like an alkene.  All of the carbon-carbon bonds in the molecule are the same.  The structure of the benzene ring allows the delocalized electrons to be spread over the ring.  The entire molecule lies in the same plane.

1. Name the parent hydrocarbon.  The parent hydrocarbon is the benzene ring 2. Add the names of the alkyl groups.

2. Number the carbon atoms in the parent hydrocarbon.  If there are two or more alkyl groups attached to the benzene ring, number the carbon atoms in the ring.  Assign position number one to the alkyl group that comes first in alphabetical order.  Then number in the direction that gives the rest of the alkyl groups the lowest numbers possible.

4. Insert position numbers. 5. Punctuate the name.