notes organic chemistry and alkanes.doc

7/28/2019 notes organic chemistry and alkanes.doc

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Notes Organic Chemistry & Alkanes

History:Vital Force Theory: organic molecules can only be created by living organisms.In 1828, Professor Whler was finishing up his post-doctoral work as a student. While working

the laboratory he succeeded in synthesizing an organic compound, urea, previously observedonly in living tissue. Whler, pictured below, made this organic compound from a non-livingchemical substance, Ammonium Cyanate. He evaporated a solution of Ammonium Cyanate toproduce Urea. Organic Chemistry has undergone a substantial change since then. There arewell over a million synthetic organic compounds. Organic Chemistry is defined as the Chemistryof Carbon and its compounds.

Carbon is a Lego

like element.I use the analogy Legos because these atoms tend to form bonds with themselves creatingdifferent shapes. Like the two below, octane on the left and a steroid precursor on the right.

The shapes are created by the chemical property of carbon to form 4 bonds.

Hybridization:The 4 bonds carbon forms is explained by Linus Paulings theory of hybridization. Carbonatoms have an electron configuration of 1s2 2s2 2p2 This configuration corresponds to thefollowing energy level diagram.

2p

2s

1s

From the looks of this diagram, carbon might form 3 bonds as appears to be nowhere for a 4thbond to form. Linus theorized that the two sublevels, s and p, in the second energy level formeda hybrid sublevel.


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This new energy level diagram is draw below

This shows four open orbitals, allowing for the formation of four bonds. Linus won a noble prizefor this theory. When carbon atoms are bonded using all single bonds the below shape formsfrom the combination of one s orbital and three p orbitals. This hybridization is named sp3.These shapes are verified by an analysis technique named X-Ray Crystallography. Thecompound is frozen then photographed with X-Rays. The resulting pictures have show thistetrahedral shape.

In this instance the carbon atom forms bonds with 4 different objects, resulting in a tetrahedralshape, is shown above picture. This shape is formed by the repulsion of the electrons whichsurround all atoms. So, if four objects are connected to a central object the farthest theseobjects can be from each other forms a tetrahedral shape. The angle between these bonds is

109.5. The blue balloon looking objects represent the orbitals where the four electrons in

carbons second energy level reside. This is the hybridization. Each of the four electrons haveequal energy and are all in the same orbitals, they have been moved to the same Shell insteadof having the 2s electrons located inside or underneath the 2p electrons.

This tetrahedral shape has also been verified by calculus. The addition of one sphere and threedumbbell shapes produces this tetrahedral shape.


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General Properties of Organic Molecules:1. Flammable2. High Vapor Pressure3. Odorous4. Covalently Bonded5. Non-Polar functional groups can change these from non-polar to polar or cause the

molecule to be bi-polar.6. Low Solubility in Water due to being non-polar as water is polar7. Rate of Chemical Reaction is Normally Slow8. Normally Found as Gasses and Liquids at Room Temp9. Non-Conductive of Electrical Current

Alkanes:The first classification for organic molecules is the most simple, the alkanet. The most simplealkanet consists of only carbon and hydrogen atoms connected by single bonds.

Alkanes are common and for the most part chemically uncreative, the chemical reaction

combustion being the major exception.

Alkanes can be found in many common substances; natural gas, gasoline, plastics

The chemical formula can be generalized as:

CnH2n+2Where n represents the number of carbons and 2n+2 equals the number of hydrogens.

Nomenclature:Nomenclature is the scientific term for naming compounds. The governing body is TheInternational Union of Pure and Applied Chemistry, or IUPAC for short. The following

statement is from their web site:

The International Union of Pure and Applied Chemistry (IUPAC) serves to advance theworldwide aspects of the chemical sciences and to contribute to the application of chemistry inthe service of Mankind. As a scientific, international, non-governmental and objective body,IUPAC can address many global issues involving the chemical sciences.

IUPAC was formed in 1919 by chemists from industry and academia. One of there mainfunctions is to objectively create rules for naming compounds in the most simplified mannerpossible. This is equivalent to creating a new language, just like English grammar, there arerules.

Even with the advent and acceptance of the IUPAC system some common names still persist,when discussing a substance the IUPAC name should be used but the common name will beaccepted by most chemical organizations.


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Alkane Nomenclature:Naming of organic structures, unlike biological classification, follows a rigid set of rules. TheInternational Union of Pure and Applied Chemistry, abbreviated IUPAC, came up with a set ofrules that follows the same standards worldwide, and is accepted among all chemists. However,common names of compounds, or names that have historical roots, are still used today for manycompounds.

The suffix for the alkane family is ane.

prefix root suffix

prefix where the substitutions are locatedroot how many carbons are in the molecules longest chainsuffix family type of functional group (alkane, alkene, alcohol, ester, etc...)

Root words are named for its number of carbons:

# of carbons root

1 meth-

2 eth-3 prop-

4 but-

5 pent-

6 hex-

7 hept-

8 oct-

9 non-

10 dec-

Example:an alkane with 3 carbons is named propaneprop for the 3 carbonsane for the family alkane (meaning all single bonds)

Formula Types:A variety of methods are used to describe a chemical compounds composition. Sometimes youwill find a the chemical formula sufficient. Other times you need to see the structure drawn out,this is referred to as the structural formula. This is a larger drawing which will show the atomsare connected. Another is the condensed structural formula, this shows the connections inaround about manner. Lastly, a more lazy form is the line structure. This simplified drawingassumes you know that carbon atoms make 4 bonds and that if you do not see a bond drawn

assume a hydrogen is occupying the undesignated bond. Also all ends and turns in the linesignify carbon atoms.


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Examples:Chemical Formula:C4H10

Structural Formula:

C CH

H

H

C

H

H

C

H

H

H

H

H

Condensed Structural Formula:CH3CH2CH2CH3

Line Structure:

Rules:1. Find the longest chain of carbons, and use this number as the base/root/parent name2. Number the chain with the end nearest the first subsistent carbon #1.3. Give the location of the alkyl subsistent by the number of the main-chain carbon that it is

attached to.4. Put the Constituents in alphabetical order (i.e. ethyl before methyl)5. Substitution Syntax:

a. between numbers and words add a dashb. between numbers add commas

Examples:

4-ethyl-octane 5-ethyl-octaneCorrect Incorrect

4-ethyl-2-methylheptane 4-ethyl-6-methylheptane 2-methyl-4-ethylheptaneCorrect Incorrect Incorrect


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Side Chain Specific Rules:4-propyloctane 4-isoproplyoctane

CH3CH2CH2CH2CHCH2CH2CH3

CH2CH2CH3

CH3CH2CH2CH2CHCH2CH2CH3

CH3CHCH3

5-butylnonane 5-secbutylnonane

CH3CH2CH2CH2CHCH2CH2CH2CH3

CH2CH2CH2CH3


CHCH2CH3CH3

5-isobutylnonane 5-tertbutylnonane


CH2

CH CH3

CH3


C

CH3

CH3CH3

Isomers:Isomerization - same molecular formula, but different structure. Also creates different propertiesfor the molecule. The number of possible isomers increases rapidly as the length of the chainincreases. These molecules are isomers of the same chemical formula.

Examples:Each of the following molecules has a chemical formula of C 4H10 but they are differentmolecules having different properties.

butane 2-methyl propane (isobutane)


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More Examples:

hexane 2-methyl pentane 3-methyl pentane

2,3-dimethyl butane2,2-dimethyl butane

not3,3-dimethyl butane

Chemical Equivalence non-isomers:When one first begins learning how to name organic compounds they typically have a difficulttime recognizing when two molecules are actually the same molecule. Depending on how thestructural formula is drawn, the same molecule may look different to the novice. Lets look atthe following two structural formulas:

C CF

H

H

H

H

H

C CH

F

H

H

H

H

These two molecules look different but they are actually the same. Single bonds are unique in

that the atoms on each side of the bond can rotate or spin around the bond. So, the differencebetween the two above molecules is simply that the carbon-carbon bond is rotated to a differentposition. At all times the fluorine and two hydrogens will be spinning about the carbon.

Lets look a this molecule a little more, by the way, its name is fluoroethane. And the reason wedo not have to say 1-fluoroethane is because no matter which carbon we put the fluorine on,that carbon will be carbon #1. The following structural drawings will all actually be of this samemolecule:

C CF

H

H

H

H

H

C CH

F

H

H

H

H

C CH

H

F

H

H

H

C CH

H

H

H

F

H

C CH

H

H

H

H

F

C CH

H

H

F

H

H

Yes, they look different, try using a molecular modeling kit and proving to yourself that yesindeed, these are all the same. You will learn that you do not have to pull any of the atoms off


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to have your model look like each structure above. You simply twist the carbon-carbon bond orflip the molecule around.

Now lets talk about a new molecule, 1,2-difluoroethane. I just added another fluorine to ourfluoroethane molecule.

C CF

H

HF

H

H

These two fluorine atoms are said to be chemically equivalent. This means that both fluorineswill react, chemically, in exactly the same manner. To recognize you look at the carbon that thefluorine is bonded to. Both fluorines have the exact same description, the fluorines are bondedto a carbon that is singly bonded to another carbon and also bonded to two other hydrogens.Since both fluorines fit this description they are chemically equivalent.

If we look at the hydrogens on this molecule we can demonstrate that they too are all chemicallyequivalent. Each hydrogen fits this description, a hydrogen bonded to a carbon, the carbon issingly bonded to another hydrogen, a fluorine and another hydrogen.

Looking back to the fluoroethane we can see that not all of the hydrogens are chemicallyequivalent. On carbon #1, there is one fluorine and two hydrogens, on carbon #2 there are onlythree hydrogens.

The following molecules have two colored atoms, these colored atoms are listed as eitherchemically equivalent or non-equivalent.

C CF

Cl

H

F

H

H

non-equivalent

C CH

F

H

C

H

H

C

H

H

F

H

H

equivalent

Br Br non-

equivalent

The easiest way to recognize chemical equivalence, is to look for symmetry. If the two atomsyou are comparing have symmetry between themselves on the molecule, they are probablychemically equivalent.


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Cyclic Alkanes:Alkanes can form compounds with themselves. By this I mean they commonly form rings.Shown below is cyclohexane. It is a hexane molecule that has come around back onto itself.The below you will find the chemical formula, structural formula and the line diagram.

C6H12

C

CC

C

CC

HHH

H

HH

HH

H

H

H

H

Sources of Alkanes:Crude oil is the main supplier. Crude oil is gently heated in a tower, this heating producesvapors, the lighter compounds to rise higher in the tower, while the heavier compounds risevery little. Collection equipment is stationed at various levels ready to remove hydrocarbons ofvarious masses. Alkanes tend to be light and very non-polar, thus they travel to the top of the

tower. Natural gas is the name given to the lightest of the alkanes, this gas is a combination ofmethane, ethane and propane. Natural gas pockets are found above large deposits of crude oil.See picture of refinement tower below.

Alkanes can also be made these synthetically in a lab, like Professor Whler, in 1828 madeurea. But his process is time consuming and expensive. Verify this yourself by pricing syntheticmotor oil verse regularmotor oil. Synthetic can easily be 15 times more expensive.


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Reactions of Alkane:Combustion:

C3H8(g) + O2(g) CO2(g) + H2O(g) + heatcomplete occurs in high oxygen environments

C3H8(g) + O2(g) CO2(g) + CO(g) + H2O(g) + less heatincomplete occurs in low oxygen environments

Halogenation:C3H8(g) + Br2(l)

uv C3H7Br(g) + HBr(g)

Dehydrogenation:C3H8(g)

Pt C3H6(g) + H2(g)

Organic Functional Groups:Organic molecules can be very complex, not only for their ability to branch off but also forspecialized groupings of atom each of which give molecules special properties. Most any ofthese groupings, named organic functional groups, can attach themselves to any organicmolecule in most any location. One molecule may have more than one of these groups givingthe molecule different properties. In our studies of Organic Chemistry the reactivity of thesefunctional groups will a main focus.


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Another Classification for Carbons: primary carbon is bonded to one other carbon:

o both of these carbons

C CH

H

H H

H

H

secondary carbon is bonded to two other carbons:

o the centerred carbon only

C CH

H

H

C

H

H

H

H

H

tertiary carbon is bonded to three other carbons:


C CH

H

H

C

H

H

H

H

C

H

H H

quaternary carbon is bonded to four other carbons


C CH

H

H

C H

H

H

C

H

H H

C

H

H H

notes organic chemistry and alkanes.doc

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