Hydrocarbon compounds

Section 22.1: Hydrocarbons

What is the relationship between carbon and organic chemistry?

• 200 years ago scientists thought that only living organisms could make carbon compounds known in living cells
• Organic means related to or made by living things
• In 1828, A German Chemist, Friedrich Wohler used an inorganic substance to make urea, and organic compound found in urine
• He showed that carbon compounds could be made by non living organisms
• Nowadays, organic chemistry is the chemistry of all carbon containing compounds

Simple organic compounds

• There are A LOT of different organic compounds
• Hydrocarbons are the simplest type of organic compounds
• Remember a compound must contain at least two kinds of elements, in a hydrocarbon these two elements are hydrogen and carbon
• Methane is the simplest examples (CH₄) - natural gas
• Can be made by bacteria as they break down dead plants, or by animals as they digest food

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What properties do you think hydrocarbons tend to have?

You have 2 minutes to discuss with your neighbor what you already know about hydrocarbons

Example might include what state they are commonly found in, or what they are commonly used for

How does the atomic structure of carbon relate to its bonding?

A carbon atom has 4 valence electrons,

How many valence electrons does a hydrogen have?

One carbon atom can form a single covalent bond with four hydrogen atoms

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Carbon structure continued

Carbon’s versatility and importance in organic chemistry stems from its ability to be able to bond with other carbon atoms

How can you represent hydrocarbon molecules?

What are some of the key properties of hydrocarbons?

• Hydrocarbon molecules are non-polar. What does this mean?
• The electrons are shared equally between between the carbon - hydrogen atoms, and the carbon - carbon atoms
• This results in only very weak van der waals forces between the molecules
• This means that “smaller” hydrocarbons tend to be gases or liquids with low boiling points
• The non polar nature also means that hydrocarbons will not form a solution with a polar substance such as water; Remember like dissolves like
• Explains why oil floats on water

Alkanes

• An Alkane is a hydrocarbon where there are only single covalent bonds
• There are only two types of bonds
• Carbon - carbon single bonds
• Carbon - hydrogen single bonds
• There are a variety of different possible arrangements, based on having a straight chain or a branched chain

Straight chained alkanes

In a straight chained alkane all of the carbon atoms bond together to form a single chain

Ethane is the simplest straight chained alkane (two carbons), propane has three carbons, and butane has 4 carbons).

The constant increment of change within the alkanes is summarized below. It is all about increases of CH₂

Properties of straight chain Alkanes

As the number of carbon atoms in the chain increases the melting and boiling points increase

The boiling points of methane, ethane, propane and butane are all below room temperature

These hydrocarbons are therefore all gases at room temperature

Naming rules for straight chained alkanes

• The International Union of Pure and Applied Chemistry set rules for the naming of compounds
• Every alkane has a name that ends with the suffix -ane
• The official and common names are the same for hydrocarbons 1 - 4
• Latin/Greek prefixes are used to name hydrocarbons greater than 4 carbon atoms

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Structural Formulas for straight chained alkanes

A complete structural formula shows all the atoms and bonds in a molecule

A condensed structural formula can show the same information, with some bonds/atoms left out

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Practice time!

Draw complete structural formulas and name the straight chained alkanes with 5 and six carbons

Will all 5 or 6 carbon hydrocarbons have this structural arrangement? What other variations/arrangements might exist?

Writing assignment

Explain why carbon is such a versatile element, and why this is important for hydrocarbon formation

Branched chain alkanes - see rules and practice problems,

Unsaturated hydrocarbons

Section 22.2

What is the difference between saturated and unsaturated?

A saturated hydrocarbon only contains single carbon - carbon covalent bonds

An unsaturated hydrocarbon contains double or triple carbon - carbon covalent bonds

These therefore do not contain the maximum number of hydrogen atoms

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Alkenes

An alkene is a hydrocarbon that contains or more carbon - carbon double covalent bonds

Shown in a structural formula as two parallel lines

Ethene is the simplest alkene (also known as ethylene)

How do you name an alkene?

Naming of alkenes is very similar to naming of alkanes, but the position of the double bond is important

To name an alkene by the IUPAC system, find the longest chain that contains the double bond.��This chain is the parent alkene.��It has the root name of the alkane with the same number of carbons plus the ending -ene.�

Naming continued….

The chain is numbered so that the carbon atoms of the double bond have the lowest possible numbers.��Substituents on the chain are named and numbered in the same way they are for alkanes.�

Alkynes

• An Alkyne contains one or more carbon - carbon triple bonds
• Other bonds may be single or double bonds
• Like alkenes they are unsaturated
• Triple bonds are shown by three parallel lines
• Ethyne is the simplest alkyne, commonly called acetylene (used in a welding torch)
• Double or triple points have no real effect on boiling points
• Boiling points of alkanes, alkenes and alkynes with the same number of carbons are all very similar

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Isomers

Section 22.3

Some hydrocarbons appear very similar

• You may have noticed that the structures of some hydrocarbons differ only in the positions of substituents or of multiple bonds.
• What is the difference between these two?

Isomers

Even though both compounds have the formula C₄H₁₀, their boiling points and other properties differ.��Because their structures are different, they are different substances.��Compounds that have the same molecular formula but different molecular structures are called isomers.

Butane and 2-methyl propane are constitutional or structural isomers

Constitutional isomers are compounds that have the same molecular formula but the atoms are joined together differently.

Constitutional Isomers

Constitutional isomers differ in physical properties such as boiling point and melting point. They also have different chemical reactivities. ��In general, the more highly branched the hydrocarbon structure is, the lower the boiling point of the isomer will be compared with less branched isomers.�

Are there other kinds of isomers?

Because molecules are three-dimensional structures, molecules with the same molecular formula and with atoms joined in exactly the same order may still be isomers.��Stereoisomers are molecules in which the atoms are joined in the same order but the positions of the atoms in space are different.�

A number of different types exist

Cis-trans isomers

Enantiomers

Cis - trans isomers

A double bond between two carbon atoms prevents other atoms in the molecule from rotating, or spinning, with respect to each other.��Because of this lack of rotation, groups on either side of the double bond can have different orientations in space.�

Cis-trans isomers, also known as geometric isomers, have atoms joined in the same order but the spatial orientation of the groups differs.��The most common example of cis-trans isomerism occurs in molecules with double bonds.

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Example

Two arrangements are possible for the methyl groups and hydrogen atoms with respect to the rigid double bond in 2-butene.�

Cis configuration

Trans configuration

Cis configuration

In the cis configuration, similar groups are on the same side of the double bond.��

Cis configuration

Trans configuration

But, when similar groups extend from opposite sides of the double bond, the isomer is in the trans configuration.��

Trans configuration

Cis-trans isomers have different physical and chemical properties.��You should be able to identify cis-trans isomers of alkenes when each carbon of the double bond has one substituent and one hydrogen.�

Enantiomers

• The second category of stereoisomerism occurs whenever a central atom has four different atoms or groups attached.�Most commonly the central atom is carbon.�A carbon with four different atoms or groups attached is an asymmetric carbon.
• In the compound shown here, H, F, Cl, and Br atoms are attached to a single carbon atom, so the carbon is an asymmetric carbon.�

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What do you notice about these two molecules? Are they the same?

• The relationship between the two molecules is similar to the relationship between right and left hands.
• Sometimes the terms right-handed and left-handed are used to describe compounds with an asymmetric carbon.
• These two molecules are mirror images of each other
• Pairs of molecules that are mirror images and not superimposable are called enantiomers, or optical isomers.�These molecules are examples of enantiomers.�

Enantiomers

Unlike other isomers, enantiomers have identical physical properties such as boiling points and densities.��Enantiomers do, however, behave differently when they interact with other molecules that have asymmetric carbons.��Many molecules in your body have asymmetric carbons, so each enantiomer can have a different effect on the body.�

Hydrocarbon rings

Section 22.4

What does the word aromatic mean?

In chemistry it does not necessarily mean smelly

Hydrocarbons do not need to have a straight or a branched chain; in some it can form a ring - a Cyclic hydrocarbon

Five or six carbon rings are the most common

Cyclic hydrocarbons can be either saturated or unsaturated

Cycloalkanes only contain single covalent bonds

These are named the