8.3 Structural Representations

Question 1

what are the 3 ways to represent organic compounds

Answer 1

Organic compounds can be represented in a number of different ways: a molecular formula, a structural formula, and a condensed structural formula.

Question 2

what does a molecular formula do

Answer 2

A molecular formula shows what kinds of atoms are present and how many of each

Question 3

what does a structural diagram do

Answer 3

A structural diagram (formula) is a two-dimensional representation of the structure of the molecule, using letters for atoms and short dashes for bonds. Bond angles are generally all represented as 90. Hydrogen molecules might be omitted

Question 4

what does a condensed structural formula do

Answer 4

A condensed structural formula is similar to a molecular formula but shows the structure more clearly. For ethane, the condensed structural formula is CH3CH3. Note that the two methyl groups are shown separately.

Question 5

what is propane

Answer 5

Propane (C3H8) is gas at room temperature but liquefies under pressure. It is commonly used as fuel for automobiles, stoves, lights and barbecues.

Question 6

what can groups of atoms around the single carbon-carbon bonds do

Answer 6

Groups of atoms in propane and other alkanes can rotate freely around the single carbon-to-carbon bonds. However, the most stable arrangements are those with the greatest separation between non-bonding atoms.

Question 7

what is butane

Answer 7

Butane (C4H10) has a boiling point of -0.5C. Like most other alkanes it is used as a fuel, perhaps most commonly in lighters and torches.

Question 8

after butane how are alkanes named

Answer 8

After butane, the alkanes are named more logically according to the number of carbons. The Greek prefixes “pent-“ (5), “hex-“ (6), “hept-“ (7), “oct-“ (8), “non-“ (9), and “dec-“ (10), precede the ending “-ane.” Thus the names pentane, hexane, heptane, octane, nonane, and decane.

Question 9

what are mixtures of alkanes used for

Answer 9

Mixtures of alkanes can be used for a variety of functions

Gasoline is pentane + decane

Kerosene is a mix of alkanes with 12-15 carbons

Lubricating oils and greases have from sixteen to twenty-five carbons.

Paraffin waxes have from twenty to thirty.

Question 10

what happens as the number of carbon atoms in a chain increases

Answer 10

In general, as the number of carbon atoms in the chain increase, the melting and boiling points also increase.

Methane, ethane, propane and butane are all gases at room temperature (25oC) and pressure, but pentane is a liquid because it’s boiling point is above room temperature

Question 11

what is the general formula for alkanes

Answer 11

The general formula for alkanes is CnH2n+2

Question 12

what alkanes do you need to know the names of

Answer 12

The first four - methane, ethane, propane, butane - do not use prefixes you are familiar with. The rest, however, do use familiar prefixes (from Unit 3 used in naming molecular compounds) that denote the number of carbon atoms

Question 13

what types of formulas do alkanes have

Answer 13

Alkanes have molecular formulas and structural formulas. A molecular formula simply counts the numbers of each sort of atom present in the molecule, but tells you nothing about the way they are joined together and so are rarely used in biochem. Structural formulas are tells you how they are joined together.

Question 14

what are the physical properties of alkanes

Answer 14

Physical Properties

1. For carbon chains of one to four C atoms ( C1, C2, C3, C4) the phase at room temperature is gas, for C5to C16is liquid, and for C17and above is solid.
2. We will learn later about special hydrocarbons called aromatics which form ring structures.All hydrocarbons which do not form these special ring structures are called aliphatic compounds.All aliphatic substances are nonpolar so they do not dissolve in water. (water is polar, and ¨like dissolves in like¨.)

Question 15

what are the chemical properties of alkanes

Answer 15

Chemical Properties

1. All bonding electrons are occupied so these compounds are said to be saturated. They are thus relatively unreactive at room temperatures making them good lubricators e.g. motor oil, and foundations for plastics.
2. At high temperatures, alkanes react vigorously with oxygen to release energy Paraffin wax for candles is an alkane (C25H52). These hydrocarbon undergo combustion reactions making alkanes valuable as fuels.
3. Can be broken into smaller units by ¨cracking¨.
4. Alkanes undergo SUBSTITUTION reactions, primarily with halogens– replacing a hydrogen with a group VIIA (or group 17) (halogen) atom

Question 16

how do you draw structural diagrams

Answer 16

• show all atoms and bonds (with or without the H atoms drawn)
• we no longer have to draw in Hydrogen atoms, the bond with no element symbolizes the H atom

Question 17

how do you draw condensed structural diagrams

Answer 17

• omits C - H bonds
• shows carbon - carbon bonds CH3- CH2- CH2- CH2- CH3

Question 18

how do you draw line diagrams

Answer 18

• the end of a line signifies a carbon atom
• hydrogen atoms are not shown for pentane1