Chemistry A Cultural Approach
Chemistry and Society

 Module 2 - Topic 1

Organic Chemistry and Polymers 

 

Objectives

 

1. Describe the general properties of organic compounds.

2. Define hydrocarbons and describe properties of alkanes, alkenes, alkynes, and aromatic compounds.

3. Classify organic compounds into families based on functional groups.

4. Use structural formulas to identify isomers.

5. Define polymer, classify polymers, and list uses.

 

Organic Chemistry and Polymers 

Properties of Organic Compounds

 

Organic chemistry is the study of compounds consisting primarily of carbon and hydrogen. Organic compounds may contain other elements including, oxygen, nitrogen, halogens, phosphorus, and sulfur.

 

Chemical compounds can be grouped into two general categories:

 

Inorganic

 

Organic

 

Examples

NaCl, H2O, MgO

 

 

Examples

CH4, CH3CH2NH2, CH3CH2OH

 

 

 

 Properties of Inorganic and Organic compounds.

 

Inorganic Compounds

-ionic or covalent bonding

-typically have high melting points and high boiling points (water boils at 100o C.

-inorganic compounds are often electrolytes and will form ions in aqueous solutions

-many inorganic compounds are soluble in water

 

Organic Compounds

-covalent bonding

-typically have low melting points and low boiling points

-generally not soluble in water

-soluble organic compounds do not form ions

-many organic compounds are flammable (gasoline)

 

Carbon has a valence of 4 valence electrons, and can form 4 single bonds, 2 single bonds and a double bond, or 1 single bond and a triple bond. Note the number of bonds around carbon in each of the following molecules. There are always 4 bonds around a carbon atom. In methane the carbon atom has 4 single bonds. In formaldehyde the carbon atom has 2 single bonds and 1 double bond, and in acetylene each carbon atom is surrounded by 1 single bond and 1 triple bond.

 

carbon has a valence of 4  

 

Formulas describe the composition and structure of molecules. Here are three methods for drawing the formula of an organic compound.

 

1. Molecular Formula

 

2. Condensed Formula

 

3. Structural Formula

 

Ethane is a hydrocarbon. It contains two carbon atoms and six hydrogen atoms.

 

Formulas for Ethane

Molecular Formula

Condensed Formula

Structural Formula

C2H6

CH3-CH3

structural formula for ethane

 

The molecular formula tells us what elements are present and the number of atoms of each element. There are 2 carbon atoms and 6 hydrogen atoms in ethane.

 

The condensed formula provides the same information as the molecular formula, and associates 3 hydrogen atoms with the first carbon and 3 hydrogen atoms with the second carbon.

 

The structural formula provides additional information about the bonding in the molecule. In the structural formula we see how atoms are bonded. Each carbon-hydrogen bond in ethane is a single covalent bond. Also the two carbon atoms are connected by a single covalent bond. The structural formula shows us there are no double or triple bonds in ethane.

 

Carbon rich organic compounds are typically nonpolar. This is a consequence of the small electronegativity difference between carbon and hydrogen atoms.

 

polar and nonpolar molecules

Nonpolar organic compounds will not dissolve in water, and are called hydrophobic (water-fearing).

 

Solubility Rule

Like Dissolves Like

 

This rule states that molecules with similar polarities are soluble, but molecules with unlike polarities are insoluble.

 

water (polar) + methanol (polar) ―› solution (like dissolves like)

 

oil (nonpolar) + vinegar (polar) ―› form layers ( oil and vinegar do not mix)

 

Nonpolar organic molecules interact with each other through weak London Dispersion forces. Consequently many organic molecules have low melting points and low boiling points.

 

This table compares the boiling point of a nonpolar organic compound (methane) with a polar inorganic compound (water).

 

Name

Formula

Boiling Point

State at 25o C

water

H2O

100o C

Liquid

methane

CH4

- 164o C

Gas

 

At room temperature (about 25o C) water is a liquid, and methane is a gas. The different properties are a consequence of the different intermolecular forces. Water molecules are held together by strong H-bonding, whereas nonpolar methane molecules attract each other with weak London Dispersion forces. The polarity of molecules plays an important role in determining the properties of chemical compounds.

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Organic Chemistry - Wikipedia

Introduction to Organic Chemistry

 

 

Organic Chemistry and Polymers 

Alkanes, Alkenes, Alkynes, and Aromatic Compounds 

 

Alkanes

Alkanes are the simplest members of the hydrocarbon family. All members of the alkane family have the general formula CnH2n+2 , where n is the number of carbon atoms. For example, an alkane with 3 carbons would have the formula C3H8.

 

Alkanes are saturated compounds. This means that each carbon atom has 4 single bonds. In ethane there is one bond between each carbon, and each carbon has three bonds to hydrogen, for a total of 4 single bonds around each carbon atom. The four single bonds satisfiy the octet rule for carbon.

 

structural formula for ethane

Ethane

 

members of the alkane family

Properties of Alkanes

- nonpolar

- insoluble in water

- soluble in nonpolar organic solvents

- low density

- low melting points

- low boiling points

 

 

Alkanes are obtained from:

petroleum

shale

coal

natural gas

 

Alkanes with ring structures are called cycloalkanes. Cyclohexane is a six-carbon ring. Each carbon in the ring satisfies its octet by forming 4 single bonds. In drawing A we see all of the carbon and hydrogen atoms in cyclohexane. To simplify the drawing we can suppress (omit) the hydrogen atoms, as in drawing B. By removing the carbon and hydrogen atoms we can futher simplify the structure as in drawing C. Drawing C is a shorthand method (line structure) for illustrating the backbone structure of organic compounds.

 

A

B

C

cyclohexane with hydrogen and carbon

 

hydrogen suppressed cyclohexane

carbon and hydrogen suppressed cyclohexane

 

 Alkanes and cycloalkanes are mostly unreactive toward other elements. This is a consequence of the nonpolar nature of the bonding in these compounds. However, alkanes and cyclohexanes do undergo combustion. Combustion is a reaction with oxygen forming carbon dioxide and water.

 

Hydrocarbon + Oxygen ―› Carbon Dioxide + Water

 

Example

methane + oxygen ―› carbon dioxide + water

 

CH4 + 2 O2 ―› CO2 + 2 H2O (balanced)

 

 

Alkenes

Alkenes are unsaturated hydrocarbons containing one or more double bonds. Members of the alkene family have the general formula CnH2n , where n is the number of carbon atoms. For example, an alkene with 2 carbons would have the formula C2H4. Ethene is the simplest member of the alkene family.

 

Ethene

structural formula for ethene

In ethene there is one double bond between each carbon, and each carbon has two single bonds to hydrogen, for a total of 4 bonds around each carbon atom. The double bond and two single bonds satisfiy the octet rule for each carbon.

 

Properties of Alkenes

- nonpolar

- insoluble in water

- soluble in nonpolar organic solvents

- low densities

 

Alkenes differ from alkanes in their reactivity. The electrons in the single bond joining two carbon atoms in an alkane are strongly held in the covalent bond. But in an alkene one pair of electrons in the double bond occupies a region of space above the carbon-carbon axis, and is easily accessible to other elements. This easy accessibility makes alkenes more reactive than alkanes.

 

Alkenes under addition reactions. Hydrogenation is an example of an addition reaction.

 

Alkene + Hydrogen ―› Addition Product

 

Example

 

ethene + hydrogen ―› ethane

 

H2C=CH2 + H2 ―› H3C-CH3

 

Alkynes

Alkynes are unsaturated hydrocarbons containing one or more triple bonds. Ethyne (acetylene) is the simplest member of the alkyne family. Ethyne has the formula, C2H2.

 

structural formula for ethyne

 

In ethyne there is one triple bond between each carbon, and each carbon has one single bond to hydrogen, for a total of 4 bonds around each carbon atom. The triple bond and single bond satisfiy the octet rule for each carbon.

 

Alkynes are highly reactive and undergo addition reactions similar to alkenes. Ethyne can be hydrogenated to alkane.

 

Example

 

Alkyne + Hydrogen ―› Alkane

 

alkyne hydrogenation

 

 

Aromatic Compounds

Aromatic hydrocarbons are ring structures with alternating single and double bonds. The physical propertes of this family of compounds is similar to alkanes and alkenes. Benzene, C6H6, is nonpolar and insoluble in water.

 

Benzene

structural formula for benzene

 

Aromatic compounds are chemically stable and do not undergo the typical addition reactions of the unsaturated alkenes and alkynes. Instead, aromatic compounds undergo substitution reactions where ring hydrogens are replaced by other groups.

 

Example

 

benzene + Cl2 ―› chlorobenzene

 

C6H6 + Cl2 ―› C6H5Cl

 

Chlorobenzene

chlorobenzene

 

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Alkanes

Alkenes

Alkynes

Aromatic

 

Organic Chemistry and Polymers

Functional Groups

 

The alkanes, alkenes, alkynes, and aromatic compounds are examples of organic families. Organic families are identified by functional groups. A functional group is a group of atoms in the molecule that contribute to the physical and chemical properties of a compound. The double bond is the functional group in the alkene family. The double bond gives the alkenes their unique chemical characteristics.

 

Organic compounds with the -OH group belong to the alcohol family. The functional group is the hydroxyl group (-OH). Here are a few members of the alcohol family:

 

methanol

ethanol

propanol

CH3-OH

CH3CH2-OH

CH3CH2CH2-OH

 

The -OH functional group is common to all members of the alcohol family. Cholesterol is a complex polycyclic organic compound. Note the -OH functional group indicated in the drawing.

 

hydroxyl group in cholesterol

 

Alkyl functions are represented by the symbol R-

 

 Methyl: CH3
 Ethyl: CH3CH2
 Propyl: CH3CH2CH2
 Butyl: CH3CH2CH2CH2

R-OH is the general formula for the family of alcohols. Examples would be:

 

 

Alcohol

Methyl Alcohol

Ethyl Alcohol

Propyl Alcohol

R-OH

CH3–OH

CH3CH2–OH

CH3CH2CH2–OH

 

 

Other families of organic compounds include:

 

Alkyl Halides

R-X where X = F, Cl, Br, I

Carboxylic Acid

carboxylic acid

Aldehyde

aldehyde

Ketone

ketone

Ester

ester

Ether

ether

 

 

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Functional Groups

 

Organic Chemistry and Polymers 

Isomers

 

Isomers are molecules with the same molecular formula, but different structural formulas.

 

Example

Count the number of carbon, hydrogen, and oxygen atoms in this molecule and write its MOLECULAR FORMULA.

 

1-propyl alcohol

1-propanol

Molecular Formula = C3H8O

 

The location of the alcohol (-OH) group is on the first carbon atom (numbering carbon atoms from the right). . The "1" in the name helps locate the position of the functional groujp. The -OH group is attched to carbon #1 in the chain.

 

Here is another alcohol.

 

2-propanol

2-propanol

Molecular Formula = C3H8O

 

In 2-propanol the -OH group is attached to the 2nd carbon in the chain. 1-propanol and 2-propanol are different substances, but they have the same molecular formula. Molecules with the same molecular formula, but different structural formula are called isomers. 1-propanol and 2-propanol are isomers.

 

Geometric isomerism is a consequence of restricted rotation around double bonds. Geometric isomers have the same molecular formula, but have a different arrangement of atoms in space.

Molecules can rotate about single bonds. For example, butane is a 4-carbon alkane. Let's focus on the single bond between carbons 2 and 3. The big gray atoms in this picture are carbon atoms, and the smaller blue atoms are hydrogen. The hydrogen atoms on the end carbons have been omitted to keep the picture simple.

 

single bond between carbons 2 and 3 in butane

Butane

Rotation is possible about the single bond between carbon 2 and carbon 3 in butane.

 

rotation around single bond in butane

 

Because of free rotation about the single bond, butane does not exist as a geometric isomer.

 

But alkenes cannot rotate around a double bond. 2-Butene is a 4-carbon alkene that looks very much like butane, except there is a double bond between carbon atoms 2 and 3.

 

2-butene

 

If we apply enough energy to rotate 2-butene around its double bond, the bond would break and the molecule would no longer be 2-butene. This means that 2-butene can exist with both carbons on the same side, or carbon atoms on opposites sides. These two molecules have the same molecular formula, but differnet orientations of the carbon atoms in space. These are geometric isomers.

 

 

 

geometric isomers of 2-butene

 

 

 

Cis and trans are labels used to identify geometric isomers. In the cis isomer the 2 end carbons are on the same side of the double bond. In the trans isomer, the 2 end carbons are on opposite sides of the double bond. The isomers of 2-butene have different properties, and different names.

cis-2-butene

 

cis butene

trans-2-butene

 

trans butene

 

 

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Isomers - Wikipedia

Isomer Construction Set

 

  

 Organic Chemistry and Polymers

Polymers

 

Polymers are substances containing a large number of fundamental units joined together to form long chains. The small units making up the chain are called monomers. Proteins and DNA are examples of biological polymers.

 

Monomer units are joined together to from polymers in a chemical reaction called polymerization. The two primary types of polymerization are:

 

1. Addition Polymerization - small molecules add to each other to make a large chain.

2. Condensation Polymerization - small molecules react to form a chain, and produce a byproduct, usually water.

 

Polyethylene is formed from a polymerization reaction involving the addition of many ethylene molecules. Heat, pressure, and catalysts are used to speed up the reaction. The lowercase n represents many ethene units. The product is a large chain of repeating alkane units. Polyethylene is a long-chain hydrocarbon.

 

ethylene polymerization

 

Branching in the polymer chain can lead to different properties.

 

LDPE (low density polyethylene) - side groups branch off the main chain

HDPE (high density polyethylene) - straight chains with no branching

 

Polymers can be classified based on their bonding properties. There are two classes of polymers.

 

1. Thermoplastics - polymer chains are held together by weak London Dispersion forces. Heating can break these weak forces allowing the polymer to be reshaped. Examples are: polyethylene, polyvinyl chloride, polypropylene, and polystyrene.

 

2. Thermosetting plastics - polymer chains are held together by strong covalent bonds. Structures cannot be reshaped by heating. Examples are: polyurethane and epoxy glue.

 

Polymer Examples

 

Name

Uses

Polyethylene (LDPE)

film wrap, plastic bags

Polyethylene (HDPE)

electrical insulation,bottles, toys

Poly(vinyl chloride), PVC

pipes, siding, flooring

Polystyrene

toys, cabinets, packaging (foamed)

Poly(methyl methacrylate)

lighting covers, signs,skylights

Poly(vinyl acetate)

latex paints, adhesives

 

Teflon is a BRAND name for polymers containing polytetrafluorethelyne (PTFE).

 

polymerization of teflon

In tetrafluoroethylene, fluorine atoms have replaced the hydrogen atoms in ethene.

 

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Chemical of the Week - Polymers

Polymers and People