Hydrocarbons
By definition (an arbitrary one that attempts to match the original definition of a molecule that could only be created by living organisms), all organic molecules contain Carbon-Hydrogen bonds. The most basic organic molecules are the hydrocarbon. Hydrocarbons, Carbon-Hydrogen polymers, form the backbone of all organic molecules. These molecules consist of a number of carbons joined together with hydrogen atoms satisfying the additional bonding requirements of the carbons. H H H H H H H H H H H | | | | | | | | | | | H-C-C-C-C-C-C-C-C-C-C-C-H | | | | | | | | | | | H H H H H H H H H H H What gives the each hydrocarbon its particular properties are the following: a) the length of the chain b) the presence of any multiple bonds in the carbon chain c) additional carbon chains branching off the main chain d) other atoms and molecules attached to the carbon chain e) the presence of carbon rings f) any resonance structures (we'll be getting to these in chapter 4)Formulas vs Structures
In hydrocarbons, as in any large molecule, there are many different ways of attaching together the atoms in a chemical formula. Where hydrocarbons differ from most of the compounds we have studied to date is that for any formula many different molecules do actually exist. It is not a matter of only one of many plausible structures being correct; many plausible arrangements actually will be found in nature. So, it is not enough to know the formula of a substance, we must know the molecular structure. A formula of a molecular substance gives the absolute number of each element in the molecule as a subscript. Formulas may be condensed so that each element symbol is written once. e.g. C5H10O2 Formulas also may be expaned to show something of how the elements are attached. e.g. CH3CH2CH2CH2COOH or CH3(CH2)3COOH Expanded formulas give you a better picture of what sort of molecule you are dealing with, but like any formula, they do NOT represent the bonds themselves. To do that, we would need to draw a structural diagram. A structure, or structural diagram, is a graphical representation of the atoms in a molecule and the covalent bonds that hold them together. They typically do not represent the 3-dimensional nature of the molecule. It just shows the ways in which the atoms are connected. This is similar to the difference between a map and a globe. The map shows which geographic features are next to each other, but does show the three-dimensional relationships between different parts of the earth (in other words, a map shows you the places you will go through to get to Australia, but does not show you that you could get there by burrowing a hole through the centre of the earth). So, even with a structural diagram, you still must imagine the 3-D structure of the molecule through your knowledge of VSEPR. Structures can be drawn in full. For example, the structure for hexane (C6H14) is: H H H H H H | | | | | | H-C-C-C-C-C-C-H | | | | | | H H H H H H The same structure also could be drawn with bond angles that better approximate the tetrahedral arrangement of bonds around each carbon.
Structural diagrams also can be condensed, either by omitting the hydrogens (you, as a
chemist, know that they are there anyway since each carbon needs 4 bonds) or we can omit
the 'C' for each carbon and just draw the bonds. In the latter case, each vertex and each
terminal end of a line segment represents a carbon.
It may seem like organic chemists are being lazy by omitting so much of the structure from
the diagram, but remember that it is the shape and length of the carbon chain that determine
the properties of the molecule. The less cluttered the diagram, the more clearly we see the
shape and length of the molecule.
Structures provide the clearest, least ambiguous indication of the nature of an organic
compound. For example, the compound CH3CH2CH2CH2COOH introduced in the section on formulas
above is much easier to visualize from the structure than the formula:
Isomers
On of the problems with formulas is that it is possible to construct more than one structure from the same formula. These different structures will have different physical and chemical properties, so we must be careful to give them different names. There are many different types of isomers. We will deal with only one type for now. Structural Isomers Structural isomers are molecules that have the same formula, but have the atoms connected together differently. For example, the the atoms in the formula C5H12 can be connected in several different ways. The isomers are shown without Hydrogens for clarity. Since they are different structures, they have different names. C C | | C-C-C-C-C C-C-C-C C-C-C | C pentane 2-methylbutane 2,2-dimethylpropane
1) Add the correct number of Hydrogen atoms to each Carbon so that it has four single bonds and
verify that each structure has the formula C5H12.
When are different drawings not really isomers?
It is possible to draw the same molecule is in different ways by rotating the whole molecle or
rotating individual bonds so that the diagram looks different, but really represents the same
structure.
For example, the following are NOT different isomers of C5H12, they are the same molecule
(2-methylbutane) rotated into different orientations.
C C
| |
C-C-C-C C-C-C-C C-C-C-C C-C-C-C
| |
C C
Similarly, the following also are NOT different isomers, they are all pentane molecules.
The only difference is that the chain has been bent differently by rotating one of the C-C bonds.
C C
| |
C-C-C-C C-C-C
|
C
As you will learn in chapter 4, single C-C bonds, (sigma bonds), can rotate around the bond axis.
Since the bonds actually are arranged tetrahedrally, this rotation causes the molecule to bend
and twist.
For example, the hexane molecule can twist itself into many different configurations:
Names, Formulas, and Structures
Naming of compounds is governed by IUPAC (International Union of Pure and Applied Chemists). The properties of a hydrocarbon molecule depend as much on the structure as it does on the particular atoms. So, the IUPAC nomenclature rules for organic molecules specify that 1) each name must apply to only one structure 2) each structure has only one valid name. So, IUPAC names specify a particular structure, NOT a particular formula. This is why different isomers of a formula have different names.
Simple Hydrocarbons
There two major classes of simple hydrocarbons: 1) Aromatic hydrocarbons contain benzene-like rings (rings with delocalized bonding orbitals) Rather than show aromatic rings as a set of resonance structures, they usually are shown with a circle inside the carbon ring. The circle represents the delocalized bonding orbitals.
2) Aliphatic hydrocarbons made up of straight chains, branching chains, or
rings that do not contain delocalized bonds
Aliphatic hydrocarbons can be divided up into two classes:
i) Alicyclic hydrocarbons have carbon chains in which the two ends of the chain
are joined to make a ring.
ii) Acyclic hydrocarbons have straight or branching carbon chains, but no rings.
Aliphatic hydrocarbons also can be classified as to whether they contain only single bonds or
if they contain one or more double bonds or one or more triple bonds. Multiple bonds alter the
hybridization state of the carbons and so they alter the geometry of the bonds around them.
Multiple bonds also alter the ability of the molecule to rotate around a carbon-carbon bond.
Alkanes: Contain only single carbon-carbon bonds.
If they are acyclic, they have the general formula CnH2n+2
If they are alicyclic, they have the general formula CnH2n
Alkenes: Contain at least one double carbon-carbon bond, but no triple bonds.
For the time being, we will only deal with alkenes with one double bond.
If they are acyclic, they have the general formula CnH2n
If they are alicyclic, they have the general formula CnH2n-2
Alkynes: Contain at least one triple carbon-carbon bond.
For the time being, we will only deal with alkynes with one triple bond.
If they are acyclic, they have the general formula CnH2n-2
If they are alicyclic, they have the general formula CnH2n-4
Naming Hydrocarbons
Alkanes, alkenes, and alkynes are named for the longest continuous carbon chain in the molecule. The number of carbons in the longest chain is indicated by a prefix in the name. number prefix of carbons 1 meth- 2 eth- 3 prop- 4 but- 5 pent- 6 hex- 7 hept- 8 oct- 9 non- 10 dec- Alkane names have the suffix -ane Alkene names have the suffix -ene Alkyne names have the suffix -yne Examples For acyclic hydrocarbons: C4H10 would be called butane C5H10 would be called pentene C8H14 would be called octyne for alicyclic hydrocarbons: The name of the ring is given the prefix "cyclo" C5H10 would be called cyclopentane C8H14 would be called cyclooctene However, the names for the above alkene and the alkyne are not complete, because the multiple bond could occur between any of several pairs of carbons. Since each of these different molecules have slightly different physical and chemical properties, the name must specify the actual configuration of the molecule.Numbering of Carbons
In order to specify the location of double bonds and (in subsequent lessons) the location of branches and other functional groups, carbons in the primary carbon chain are numbered. How do we know where to start numbering (which carbon will be number 1)? In an acyclic (non-ring) hydrocarbon, one of the terminal carbons must be number 1. In an alicyclic (ring) hydrocarbon, we could pick any carbon to be number 1. However, there are rules that specify how the numbering must take place. Writing Names for Non-branching Hydrocarbons Acyclic (straight chain) Hydrocarbons a) Acyclic hydrocarbon numbering can start at either end. b) The location of a multiple bond is given by lower of the two carbons around it. c) Numbering always starts at the end that gives the multiple bond the lowest number. d) Branches off of the main chain are numbered according to the which carbon on the main chain to which they are attached. e) If no multiple bonds are present, the carbons in the main chain are numbered so as to give the branches the lowest possible set of numbers. f) The name is preceeded by a number indicating the position of the multiple bond (if present). g) The number is separated from the rest of the name by a hyphen. h) Where only one position is possible, the number is omitted from the name. Carbon Numbers: 9 8 7 6 5 4 3 2 1 1 2 3 4 5 6 7 8 9 1 2 3 H H H H H H H H H H H H H H H | | | | | | | | | | | | | | | Structure H-C-C-C-C-C-C-C=C-C-H H-C-C-C-C=C-C-C-C-C-H H-C=C-C-H | | | | | | | | | | | | | | | | | | | | | H H H H H H H H H H H H H H H H H H H H H Name: 2-nonene 4-nonene propene (1-propene would be superfluous) If the carbons were numbered from the opposite end we would get: 7-nonene 5-nonene 2-propene In each of these cases, the carbons were numbered starting at the wrong end, the result of which is that the number of the multiple bond is not as small as it could be. Alicyclic Hydrocarbons i) If the hydrocarbon in alicyclic, the numbering always starts one one side of the multiple bond so that the multiple bond is between carbon number 1 and 2. H H H H \| |/ 5C-C6 4/ \7 H-C-H H-C-H | | H-C-H H-C-H 3\ /8 2C=C1 / \ H H cyclooctene It is not 1-cyclooctene. The multiple bond must be between C1 and C2, so the multiple bond must occur after the 1st carbon. Since there is only one allowable position, we can omit (and must omit) the 1 from the name. Questions 2) Draw all of the structural isomers for C7H16. 3) Name the following hydrocarbons and give the correct formula for each. a) C-C-C b) C-C-C-C-C=C-C-C c) C-C
C-C-C
d) C-C-C-C-C
|
C
e) C-C
| |
C-C
4) Draw the correct structure for the following names and give the correct formula. a) cyclopentane b) 2-butyne c) 3-hexene d) heptane
Branching Hydrocarbons
As we have said, there is more than one possible structure for a given formula. Different structures that have the same formula are called isomers. One way to create isomers is to remove carbons from the main chain of a hydrocarbon and attach them as side branches from carbons in the middle of the chain. For example, consider C6H14 Each of the following structures has this formula (the hydrogens are omitted for clarity) C-C-C-C-C-C C-C-C-C-C C-C-C-C-C C C | | | | C C C-C-C-C C-C-C-C | | C C hexane 2-methylpentane 3-methylpentane 2,2-dimethylbutane 2,3-dimethylbutane Since each molecule has a different structure, it must have a different name.Naming Branching Hydrocarbons
The Longest Chain The fundemental name of any branching hydrocarbon is the name of the longest continuous carbon chain. In alkenes or alkynes, the longest chain must contain the multiple bond. For example: C-C-C-C-C=C C-C-C-C-C=C | | C-C-C-C C-C-C-C The straight chain is 6 carbons, but this is not the longest chain. The longest chain is 8 carbons (in red), but this chain does not contain the double bond. The longest chain with the double bond is 7 carbons (in blue). So, this would be a heptane, not an octane or a hexane. The correct name is: 3-n-propyl-1-heptene Naming Branches Each branch coming off the main chain is given a name that depends on how many carbons are in the branch. Branch names have the suffix -yl to indicate it is a branch. They have the same prefixes as any other hydrocarbon to indicate the number of carbons in the branch. The following molecule has 2-carbon branches (ethyl) and 1-carbon branches (methyl) C-C C | | C-C-C-C-C-C-C-C-C-C | | C-C C 4,5-diethyl-6,7-dimethyldecane Rules For Naming Branching Hydrocarbons 1) Find the longest chain. Use this for the prefix of the base name. Identify any multiple bonds so as to chose the correct suffix for the base name. 2) The base name is preceeded by the names of each type of branch in the molecule. 3) Branch names are listed in alphabetical order. 4) Identify the carbon numbers of the main chain to which each branch is attached. List these numbers in front of the name of each branch type. Branches Numbers always are separated by commas. 5) Sets of numbers always are separated from the branch names by hyphens. 6) Use greek prefixes for the branch names to indicate the number of branches of each type. 7) The last branch name is run into the name of the base chain. In the example above: The longest chain is 10 carbons, hence the "dec" prefix. There are no double or triple bonds, hence the suffix "ane" Thus, this is a "decane" We started numbering the carbons of the main chain with number 1 at the left. "4,5" tells us that there is an ethyl group comming off of the 4th and 5th carbons in the chain. the "di" infront of "ethyl" tells us that there are two ethyl branches in this molecule. "6,7" tells us that there is a methyl group comming off of the 6th and 7th carbons in the chain. the "di" in front of the "methyl" tells us that there are two methyl branches in this molecule. The advantage of the seemingly complicated name: 4,5-diethyl-6,7-dimethyldecane is that it allows the reader to unambiguously reconstruct the structure of the molecule.Other Considerations 1) When there are no multiple bonds, carbons are numbered starting at the end that gives the smallest branch numbers. In the case of the above molecule, we would have got the same set of numbers starting with carbon 1 at either end of the chain. 2) When the same set of numbers would be achieved starting at either end, we start at the end that gives the longer branch the smaller numbers. In this case, we started at the left end so that the ethyl branches got the smaller numbers. For the structure: C-C C | | C-C-C-C-C-C-C-C-C-C | | C-C C we would start numbering from the right to get: 5,6-diethyl-3,4-dimethyldecane because this gives a lower set of numbers than would be the case if carbons were numbered from the left; which would give the name 5,6-diethyl-7,8-dimethyldacane. 3) If multiple bonds are present, the carbons are numbered to give the multiple bond the lowest number and the branches get what ever number they get as a result. For example: C-C=C-C-C-C-C is 6-methyl-2-heptene (not 2-methyl-5-heptene) | C Potential Problems in Identifying the Longest Chain 1) The longest chain is not necessarily the straightest chain as the molecule is drawn. e.g. C-C-C-C-C-C-C is the same structure and will have the same name as C-C-C-C-C-C | | C C | C 3-methylheptane 3-methylheptane The longest chain in each case is 7 carbons. It does not matter that the longest chain makes a change in direction. In fact, the left-hand structure could be turned into the right hand structure just by rotating around the bond before the branch. Remember, these bonds are not at right angles, but are tetrahedrally arranged so that C / C C C C \ / \ / \ / C C C can become C / \ C C C C \ / \ / \ C C C just by rotating the molecule around the bond that is highlighted.
Examples e.g. What is the name for: C-C-C | C-C-C-C-C-C-C-C If you guessed 2-propyloctane, you did not find the longest chain. The longest chain is nine carbons long, which gives us the name 4-ethylnonane It is not that 2-propyloctane does not describe the molecule unambiguously, but IUPAC rules state that there only should be one name per structure (to avoid confusion) and they also state that the longest chain should be reflected in the base name. e.g. What is the name for: C-C-C-C-C | C It is not 1-methylpetane. You can not have a 'branch' on the end carbon of a chain. Adding a carbon to the end of a chain does not make a branch, it just makes the chain longer.
Branching Cyclic Hydrocarbons
When a cyclic hydrocarbon has a double or triple bond, you have little choice as to where to start numbering the carbons. However, you should number the carbons clock-wise or counter clockwise depending on which direction gives you the smaller branch numbers. H H H H \| |/ C-C / \4 /H H-C-H H-C-C-H | | \H H-C-H H-C-H \ /3 1C=C2 / \ H H would be 4-methylcyclo-octene (start at lower left and go counter clockwise) not 7-methylcyclooctene (clockwise gives a higher branch number) not 3-methylcyclooctene (the multiple bond must be between C1 and C2)
When there are no double or triple bonds, start numbering the carbons at a carbon
and in a direction that gives you the smaller branch numbers. When there are
choices with the same set of numbers, choose the numbering scheme that gives the
larger branches the smaller set of numbers.
H H H H
\| |/
C-C
/ \3 /H
H-C-H H-C-C-H
| | \H
H-C-H H-C-H
\ /2
C-C1
/ \
H H-C-H
|
H
1,3-dimethylcyclooctane
Start with the lower right carbon and go counter clockwise or start with the upper
right and go clockwise. You get the same set of branch numbers in either case.
Questions
5) Write the IUPAC name for the following structures.
a) C-C-C-C-C-C-C
| | | |
C C C C
| |
C C
b) C C
| |
C-C-C-C-C
|
C
6) Draw structures for the following names:
a) 2,2,4-trimethylhexane
b) 1,1,2-trimethylcyclopentane
c) 2,3-dimethyl-2-butene
d) 1-ethyl-2,3-dimethylcyclohexene
7) Name the following structures:
8) a) Draw the structure for the name 3-propyl-6-methyl-4-hexene
b) List all the things that are wrong with the name 3-propyl-6-methyl-4-hexene
c) Give the correct name.
9) Give the correct IUPAC names for the following:
Longer Branches
In this course, we will only consider branches as large as butyl. However, even with branches this short, there are several possible ways in which the branches can be attached to the main chain. Each way will produce different structures with different properties, so each must be given a different name. C-C C-C-C C-C-C C | | | | C C-C-C C C-C-C-C C C-C-C | | | | | | C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C n-propyl isopropyl n-butyl s-butyl isobutyl t-butyl Questions 10) Draw structures for the following: a) 5-s-butyldecane b) 2-n-propyl-3-isobutylhexene 11) Give the correct IUPAC name for the following: a) C-C-C-C C | | C-C-C-C-C-C-C b) C-C-C-C-C-C-C-C-C | C | C-C-CAromatic Hydrocarbons
Aromatic hydrocarbons are distinguished by having rings with delocalized bonding orbitals. The most familiar is benzene, a 6 carbon ring: C6H6 Benzene rings are named just like cyclohexanes, except that the name of the ring is "benzene" Examples:
These would be names (from left to right) benzene, methylbenzene, 1-ethyl-3-methylbenzene.
In the cases where benzene rings are attached to carbon chains that are too complicated to
name as branches, the benzene ring is named as a branch instead. When benzene rings are
named as branches they are called phenyl rather than benzyl.
Questions
12) Draw structures for the following:
a) 1,3-dimethylbenzene
b) 4-phenyl-2-pentene