Unit 2 Packet - Wikispaces2+Packet.pdf · Unit 2 Packet: Alkanes: Sources, Uses, Nomenclature ... Honors Organic Chemistry 37 Alkanes: Nomenclature Alkane names: n 1 Methane 2 Ethane

Name: _______________________________

Unit 2 Packet: Alkanes: Sources, Uses, Nomenclature

and Conformational Analysis

Honors Organic Chemistry

33

Key Terms For Unit 2

General

Fractional Distillation

Petroleum

Catalytic Cracking

Oxidation Number

Nomenclature

Alkane

Alkyl Group

Substituent

“iso”

“sec”

“tert”

Primary - 1°

Secondary - 2°

Tertiary - 3°

Conformational Analysis

Sawhorse Projection

Newman Projection

Conformations

Eclipsed

Staggered

Torsional strain

Angle strain

Anti and Gauche

Boat and Chair

Equilibrium

Transition State

Ring Flipping

Axial and Equatorial


34

Alkanes: Occurance and Uses

Alkane is the term associated with any ______________ hydrocarbon, meaning any compound

composed of carbon and hydrogen that contains no double or triple bonds (unsaturation).

The primary sources for alkanes ranging from 1 – 40 carbons are ____________ and _________

_____.

Natural Gas:

1. Primarily methane (~95%), with the additional ~5% coming from ethane, propane

and butane.

2. Natural gas that is mined in Pennsylvania has methane, ethane and propane in a ratio

of 12 : 2 : 1.

Petroleum:

1. Can range in size from 1 – 40 carbons.

2. Petroleum is separated into different fractions based upon their differences in boiling

points through a process called ___________ ____________. The individual

fractions tend to have similar properties and are more useful than crude oil.

3. Pennsylvania crude oil contains large amounts of long-chain alkanes (C20 – C40),

which have high melting points. They must be removed from the oil to prevent

solidifying during cold weather and clogging oil lines. One such operation, in Oil

City, PA, removes these high MW alkanes and sells them as paraffin wax and

Vaseline.

Fractional Distillation:

1. The crude oil mixture is heated to a high temperature

(~380°C).

2. The mixture boils, forming vapors

3. The vapor enters the bottom of a large fractional

column that is filled with trays or plates

a. The trays have many holes or bubble caps (like a

loosened soda cap) in them that allow vapors to

pass through

b. The trays increase contact time between the vapor

and the liquids in the column

c. The trays collect liquids that form at various heights

in the column

d. There is a temperature __________ that forms

across the column (hot bottom, cooler top)

4. The vapor rises in the column

5. As the vapor rises, it cools into the trays

6. When a vapor reaches a point in the column where its

temperature is equal to its boiling point, it condenses to

form a liquid.

7. The trays collect the liquid fractions, which may pass to

condensers to cool them further, and eventually pass

them onto storage tanks for further processing.


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Principle Uses of Alkanes as Fuels

Methane – 95% of natural gas – also found in marsh gas and in the digestive tracts of mammals

Ethane – natural gas, also dissolved in solution of petroleum

Propane – LP gas (liquefied petroleum) for use in grills, small amounts in natural gas

Butanes – two isomers: n-butane (B.P. = 0.5°C) and isobutane (B.P. = -10°C)

Pentanes – (3) found in petroleum ether and gasoline fractions – used as solvents and fuels and

in the synthesis of other compounds

Hexanes – (5) found in petroleum ether and gasoline fractions – used in solvents and fuels

Heptanes – (9) most important is n-heptane. Found in petroleum and also obtained from the

resin of the Jeffrey Pine in the Sierra mountains. Some turpentines are 90% heptane. N-heptane

exhibits bad “engine knock” when used in internal combustion engines. Consequently, for the

purposes of gasoline octane rating, n-heptane is assigned a value of ”0.”

“Jeffrey Pine wood is similar to Ponderosa Pine wood, and is used for the same

purposes. The exceptional purity of n-heptane distilled from Jeffrey Pine resin led to n-

heptane being selected as the zero point on the octane rating scale of petrol.

As n-heptane is explosive when ignited, Jeffrey Pine resin cannot be used to make

turpentine, after the landmark Supreme Court ruling, J. Pine v. Your Turpentine Co. in

1896. Before Jeffrey Pine was distinguished from Ponderosa Pine as a distinct species in

1853, resin distillers operating in its range suffered a number of 'inexplicable' explosions

during distillation, now known to have been caused by the unwitting use of Jeffrey Pine

resin.”

-Wikipedia, Jeffrey Pine

Octane – (18) most important is 2,2,4-trimethylpentane which is assigned an octane rating of

“100”. It is made from butane and butane fractions of gasoline or natural gas.

Higher MW Alkanes:

Hexadecane (cetane) – C16H34 is an excellent diesel fuels and is used to rate diesel fuels

n-heptacosane – C27H56 occurs in beeswax and in small quantities in American tobacco leaves

n-hentriacontane – C31H64 also occurs in beeswax and tobacco. It is also found in the waxy

coating on many green leaves.

**Alkanes with odd numbers of carbon atoms C25-C37 are solids at room temperature and

occur in many plant and insect waxes**

http://en.wikipedia.org/wiki/Heptane

http://en.wikipedia.org/wiki/Petrol#Octane_rating

http://en.wikipedia.org/wiki/Petrol

http://en.wikipedia.org/wiki/Explosive

http://en.wikipedia.org/wiki/Turpentine


36


37

Alkanes: Nomenclature Alkane names:

n 1 Methane

2 Ethane

3 Propane

4 Butane

5 Pentane

6 Hexane

7 Heptane

8 Octane

9 Nonane

10 Decane

n 11 Undecane

12 Dodecane

13 Tridecane

14 Tetradecane

15 Pentadecane

16 Hexadecane

17 Heptadecane

18 Octadecane

19 Nonadecane

n

20 Eicosane

21 Heneicosane

22 Docosane

23 Tricosane

24 Tetracosane

25 Pentacosane

26 Hexacosane

27 Heptacosane

28 Octacosane

29 Nonacosane

n

30 Triacontane

31 Hentriacontane

32 Dotriacontane

33 Tritriacontane

40 Tetracontane

50 Pentacontane

60 Hexacontane

70 Heptacontane

80 Octacontane

90 Nonacontane

100 Hectane

132 Dotriacontahectane

IUPAC Rules:

1. Select the longest continuous chain of carbon atoms as the “parent chain”

a. If two chains of equal length compete for parent chain:

i. Choose the one with the greater number of substituents

ii. If each chain contains the same number of substituents, choose the one

with the lowest numbers (see #2).

2. Number the parent chain for the purpose of assigning numbers to any substituent groups

a. The direction of numbering is chosen to give the lowest possible numbers to

substituents

b. The lowest number series is the one that contains the lowest number on the

occasion of first difference

c. Each and every substituent must be assigned a number

3. Name the compound by using the prefixes appropriate for the substituents before giving

the name of the parent chain

a. Use retained common names for alkyl group substituents wherever possible

b. Halogen substituents are named by changing “ine” to “o”

c. All substituents are named in alphabetical order, regardless of numbering

d. Identical unsubstituted radicals/substituents are prefixed to indicate the total

number of them.

e. The prefixes mono, di, tri, tetra…etc. and sec- and tert- do not count for

alphabetical order (unless they are within parenthesis, where they do). Iso, neo

and cyclo do count.

**Between any # and letter there must be a dash, “-“**

**Between any two #s there must be a comma, “,”**


38

Alkyl Groups

Alkyl groups are saturated substituents (attached to the main chain) that contain only carbon and

hydrogen. They are named from the alkane with the same number of carbon atoms. In

comparison to the alkanes that they are derived from, alkyl groups have one fewer hydrogen

atoms corresponding to the carbon that serves as the point of attachment to the parent chain.

Certain common names of some simple alkyl groups have been retained for IUPAC use.

Alkane – Common Names Alkyl Groups Possible

Methane Methyl

Ethane Ethyl

Propane n-propyl

Or

isopropyl

Butane n-butyl

Or

Sec-butyl

Isobutyl

Isobutane

Or

Tert-butyl


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Alkyl Groups (cont.)

Alkane – Common Names Alkyl Groups Possible

n-pentane n-pentyl

isopentane Isopentyl

Or

Tert-pentyl

neopentane neopentyl

n-hexane n-hexyl

Isohexane isohexyl


40

Carbon Classification

Primary - 1°

Tertiary - 3°

Quaternary - 4°

Secondary - 2°

Primary: Attached to only one other carbon atom

Secondary: Attached to two other carbon atoms

Tertiary: Attached to three other carbon atoms

Quaternary: Attached to four other carbon atoms

Alkyl Groups – Notes:

1. The “n” (normal) designation is used for all straight-chain (un-branched) saturated

hydrocarbons regardless of chain-length.

2. The “iso” designation is retained for use on all alkanes/alkyl groups of 6 carbons or

less. An “iso” alkane is a straight chain with only 1 methyl group attached to the 2nd

carbon from the end. For an alkyl group to be designated “iso”, the point of

attachment must be the terminal carbon at the end of the chain that is opposite to

where the methyl group is attached.

3. The “sec” and “tert” designation for the butyl groups comes from the designation of

the carbon at the point of attachment as being either 2° or 3°. Although various

pentyl and hexyl groups may have 2° or 3° carbons, the “sec” and “tert”

designations are reserved for the butyl substituents.

**with the exception of tert-pentyl**

Isomers – Interesting info:

Molecular Formula Number of Structural Isomers

C4H10 2

C5H12 3

C6H14 5

C7H16 9

C8H18 18

C9H20 35

C10H22 75

C11H24 159

C14H30 1858

C20H42 366,319

C30H62 about 4.11 x 109

C40H82 about 6.25 x 1013


41

Alkane Nomenclature – Worksheet #1

I. Give the Structural Formula of the following compounds:

a. 4 – ethyl – 2,4 – dimethylheptane

b. 2,2,3,3 – tetramethylpentane

c. 4 – ethyl – 3,4 – dimethylheptane

d. 2 – chloro – 3,4 – dimethyldecane

II. Draw out the structural line formula and give IUPAC names for the following

compounds:

a. (CH3)2CHCH2CH2CH3

b. (CH3)3CCH2C(CH3)3

c. (CH3)2CHCH2CH2CH(C2H5)2

d. CH3CH2CH(CH3)CH2CH(C3H7)CH2CH3

e.

f.

III. Write the IUPAC names and common names for the following compounds:

a. CH3CH(CH3)CH2Cl

b. CH3CHBrCH2CH3

c. (CH3)2CHCH2CH2Br

d. CH3C(CH3)3

e. CH3CH2CBr(CH3)2

f.

g.


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I. Give the IUPAC names for the following compounds:

a.

b.

c.

d. II. Give the common name for the following:

e.

f.

g.

h.

i.

1.

2.

3.

4.

5.


43


II. Give the IUPAC names for the following compounds:

a.

b.

c.

d.

e.

III. Write Common Names for the

following compounds


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IV. Draw the following structures:

a. 2-bromo-3-cyclopentyl-6-isopropyl-7-methylnonan-5-one

b. 1-(2-bromo-2-methylpropyl)-2-isopropyl-3-methylcyclohexane

c. 7-hydroxydodecanal

d. 2,2-diethylpropandial

e. cycloheptylmethanoic acid

f. t-pentyl chloride


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V. Draw the following structures:

a. trans-1-t-butyl -3-isopropylcyclopentane

b. pentandioic acid

c. cyclopentyl iodide

d. hexafluoroethane

e. cyclobutylcyclohexane

f. 4-t-butyl-6-ethyl-6,7-dihydroxydecanal


46


VI. Write the IUPAC names of the following structures:

a.

b.

c.

d.

e.

f.

g.


47

Naming Practice


48

Group Nomenclature Practice


49

Acyclic Alkanes: Conformational Analysis

For a moment, let’s consider some important properties of alkanes.

1. Alkanes burn – One of the most important applications of alkanes are as ________.

Natural gas (methane, ethane and propane), LP gas (primarily propane), lighter fluid

(butane), gasoline (C5 – C10), kerosene (C11 – C13) and diesel fuel (C14 – C18) are

all important fuels.

2. Alkanes the starting point for many ___________ – the vast majority of man-made

plastics, synthetic fibers and materials are made from petroleum starting materials.

3. Alkanes are of minimal use to organic chemists – Alkanes are made entirely of

carbon and hydrogen connected by sigma bonds, involving no ____________ groups.

Consequently, alkanes are, for the most part, fairly un-reactive. There are a small

handful of reactions that are useful for converting alkanes into more chemically

useful compounds.

4. Alkanes are non-polar – Again, because alkanes are made up of carbon and

hydrogen (between which there is relatively little difference in electronegativity),

they are non-polar. This means they dissolve in non-polar solvents and try to avoid

polar solvents.

The most interesting things about alkanes are not what you can see, but what you cannot see.

Let’s consider what several small alkanes are like on the atomic scale.

Methane:

Methane, the simplest alkane, undergoes very few interesting reactions. On a close up view, it is

tetrahedral, having bond angles of 109.5°.

Ethane:

Like methane, ethane has 109.5° bond angles, more or less. However, ethane has a new feature

that is apparent by looking at a 3-D model. Unfortunately in the venue of a packet, I only have

2-D available. Because the two carbons in ethane are held together by a sigma bond, they are

free to rotate around the carbon-carbon bond, creating an infinite number of possible

_______________. To help visualize this, let’s look at two traditional representations of ethane.

Sawhorse Projections:

Staggered Eclipsed


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Sawhorse Projections (cont):

The sawhorse projections of ethane shown above show two of the possible conformations of

ethane, eclipsed and staggered. These terms have to do with the relationships of the hydrogens

on the two carbons with one another. An eclipsed conformation has hydrogens that would

overlap if the molecule were viewed down the carbon-carbon bond axis, while a staggered

conformation has hydrogens that would be perfectly offset when viewed down the same bond

axis.

One problem with the sawhorse projection is the optical illusion that it presents. After a while,

you can see the sawhorse projection with either carbon being the one that is closer to you in 3-D

perspective. For our purposes, let’s agree that the one on the lower left is closer.

Newman Projections:

Back Carbon

Front Carbon

Ethane – Staggered

Newman Projections

Ethane – Eclipsed

The Newman projections shown above again demonstrate the staggered and eclipsed

conformations of ethane. The eclipsed conformation involves some cheating so that it is possible

to see what is attached to each cabinet.

Who cares?

Why even talk about this? Is there any difference between ___________ and ____________ or

anything in between? It may be difficult to see this by looking at a 3-D model, because you will

likely have no problem spinning ethane about the carbon-carbon bond. It is important to

remember however, that the bonds represented by sticks on a model are actually made up of

shared pairs of electrons, and electrons repel other electrons. It turns out that ethane “prefers”

the staggered conformation as opposed to the eclipsed conformation.

Obviously ethane doesn’t have feelings or preferences, but there is an energy difference between

the two conformations. Eclipsed conformations are higher in energy, and therefore unfavorable

as compared to staggered. The energy difference between the two conformations is about 3


51

kcal/mol, or about 12 kjoules/mol. This energy hike in the eclipsed conformation is due to a

force called torsional strain. The kcal (or, Calorie with a capital C), is the same one that used to

measure the energy content of food.

For reference, a mole of carbon-carbon bonds is worth about 100 kcal, which is about the amount

of energy required to boil off 1.5 quarts of water. For a compound hanging around at room

temperature, there is enough energy available to do most things worth 15 kcal/mol or less. So

the 3 kcal/mol difference in the conformations of ethane is pretty trivial. Even so, it is an

instructive example of energy differences.

Energy Diagrams:

Chemists often draw energy diagrams, which are pictures used to demonstrate how energy

changes over the course of some process. In this case, the process that we will observe is the

rotation of the ethane molecule. Notice the energy oscillation as the molecule is rotated about

the C-C bond.

Propane:

The Newman projections of propane look slightly different. Now, the eclipsed conformation has

a hydrogen overlapped with a methyl group. This involves a slightly higher energy difference,

3.3 kcal/mol.

Butane:

Butane’s Newman projections are more interesting still. Now, two new possible conformations

exist, anti and gauche. Notice on the diagram below that there are energy differences between

the various eclipsed conformations as well as differences between anti and gauche

conformations.


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Anti Gauche Methyl-Methyl Methyl-hydrogen

Eclipsed Eclipsed

Energy Diagram

Pentane and higher:

The terminal bonds of pentane will be essentially equivalent to those in propane and butane,

while the central bonds will behave more or less like the central bond of butane, rotating freely

but preferring anti conformations. This is why we often draw a line angle representation of the

molecules as a long zig-zag.


53

Cycloalkanes

Compounds with rings are very similar to their acyclic counterparts: they do very little chemistry

of interest, because they have no functional groups. Cyclic compounds are named just like their

acyclic counterparts, with the prefix “cyclo” is added for distinction.

Cyclopropane:

Attempting to make cyclopropane with a model kit can be a frustrating endeavor. Chances are,

you will either not be able to make cyclopropane without breaking your carbons/bonds, or you

will be able to make the model with significant difficulty. The reason for this difficulty is that

you are taking sp3 carbons designed for 109.5° bond angles and using them to make 60° bonds.

This results in what is called angle strain. How can we measure how much energy is associated

with this angle strain? One method is to burn it.

Heat of Combustion:

Burning any substance releases heat, which can be measured using an apparatus called a

calorimeter. When cyclopropane is burned in a calorimeter, it releases 500 kcal/mol of energy.

To determine the energy that arises from angle strain, however, we need to be a little bit creative.

One guess would be to compare cyclopropane to propane, but this attempt is foiled by the fact

that cyclopropane is C3H6 and propane is C3H8. Propene has the same formula, but it has a

carbon-carbon double bond, which is higher in energy than a sigma bond. Here is the creative

way around this issue:

Heat of Combustion

Pentane CH3-CH2CH2CH2-CH3 845.2 kcal/mol

Ethane - CH3-CH3 - 372.8 kcal/mol

Cyclopropane CH2CH2CH2 472.2 kcal/mol

(w/o strain)

Cyclopropane (actual) 500.0 kcal/mol

- Cyclopropane (w/o strain) - 472.2 kcal/mol

Strain Energy 27.8 kcal/mol

This 27.8 kcal/mol of strain energy comes from 6 kcal/mol of unavoidable eclipsing hydrogens

(torsional strain) and about 22 kcal/mol of angle strain

Cyclobutane:

Cyclobutane also undergoes angle strain, with sp3 carbons being forced into 90° bond angles.

The hydrogens in cyclobutane are also totally eclipsed. Total strain = 26 kcal/mol.


54

Cyclopentane:

A regular pentagon would have 108° bond angles, but it would also have eclipsed hydrogens.

Cyclopentane flexes a bit to avoid some torsional strain, at the expense of gaining some angle

strain. Total strain = 6 kcal/mol.

Cyclohexane:

A regular hexagon would have 120° bond angles. This would pose an angle strain and torsional

strain mess. Cyclohexane, however, is large enough that it is not forced to be flat. It can orient

itself in a strain-less conformation, where no hydrogens are eclipsed and all bond angles are

109.5°. The lowest possible energy conformation of cyclohexane is called a chair, the other

common conformation is called a boat. We will discuss the implications of these further.

Chair Conformation

Boat Conformation

Why is the boat more uncomfortable than the chair? Eclipsed hydrogens on the side and

hydrogens that nearly touch on the top.

Boat Conformations – Shows eclipsed hydrogens (left) and hydrogens that compete for space

Even though cyclohexane is more comfortable as a chair, it does not always exist in the chair

form. A sample of cyclohexane forms an equilibrium between the chair and boat conformations,

where a certain sample of cyclohexane always exists in each form at any given time.

At room temperature, the equilibrium equation for this reaction is as follows.

Keq = 3.97 x 10-5 = ][

][

chair

boat=

1

1097.3 5x

In other words, at room temperature there is 1 boat for every 25,159 chairs.


55

Ring flipping:

Cyclohexane can oscillate from a chair to a boat to a different chair form by the process of

flipping. You can do this yourself on a 3-D model. We know that boat is higher in energy than

chair, but if you try this on your own, you will find that the in-between state is even tougher to

achieve, and therefore higher in energy. This is visible in the graph below. The energy peak

shown with the ‡ is called a transition state.

The point here is that there are two different types of positions for the hydrogens in a chair –

axial, where the hydrogens are positioned vertically on an axis, and equatorial, where the

hydrogens are positioned horizonally off of the molecule. When a chair undergoes flipping, the

hydrogens that are axial in the first conformation become equatorial in the second conformation,

and vice versa.

Equatorial Axial

For cyclohexane, there is no preference between either chair conformation because they are

essentially the same. However, when the ring has a substituent attached, whether the substituent

is axial or equatorial becomes energetically important.

On a 3-D model, try replacing a hydrogen with a methyl group to form methycyclohexane.

Unfortunately, your model does not do justice to the issue and you probably cannot determine a

preference between axial and equatorial for the methyl group. Take a look at the two different

conformations shown below, paying special attention to the space fill diagrams on the right.


56

Stick Model Space-Fill Model

Methyl

Group is

Axial

Methyl

Group is

Equatorial

The orientation of methycyclohexane where the methyl group is axial has strain associated with

the methyl group bumping into the hydrogens that are axial on the carbons that are beta (2 away)

to the methyl group. The equatorial orientation is therefore favored, because it does not have this

issue. As the size of the group substituted on a cyclohexane increases, the preference for

equatorial increases drastically.

Methycyclohexane – 95% equatorial

t-butylcyclohexane – 99.99% equatorial

Disubstituted Rings:

There are 4 different isomers of dimethylcyclohexane. 1,1; 1,2; 1,3; and 1,4. The latter three all

have cis and trans, which each boast two different chair forms. Which is more stable, cis or

trans? Well, it depends on where the methyl groups are substituted. Let’s look at 1,2 –

dimethylcyclohexane:

Cis trans

In either cis form, one of the groups is equatorial (good), and the other is axial (bad).

Cis Forms:


57

In the trans form, one chair has both methyl groups equatorial (great!), and the other has both

methyl groups axial (real bad).

Trans Forms:

The trans form of 1,2 – dimethycyclohexane is much more stable than the cis form because it

can be a conformation that is lower in energy (both equatorial).

Question: Is trans disubstituted always better?

Answer: No, the trans form is preferred for 1,2 and 1,4, while cis is preferred for 1,3. The

preferred form is always the one that allows for both substituents to be equatorial.


58

Conformational Analysis Worksheet

1. Draw the Newman projection for the following conformations of 1,2-

dibromoethane:

a. Anti

b. Gauche

2. Draw an energy diagram for the rotation of 1,1,2-tribromoethane:

3. To determine the strain energy in cyclobutane, you need the combustion energy

information for what set of compounds?

4. How would you analyze the above set of energies to find a strain energy value for

cyclobutane?


59

Conformation Analysis Worksheet (cont.)

5. Draw the stable chair form for trans-1,4-dimethylcyclohexane. Draw a circle

around the equatorial hydrogen atoms and a box around the axial hydrogen

atoms.

6. For cis-1,3-dimethylcyclohexane, draw:

a. The most stable chair

b. The least stable chair

7. Isopropylcyclohexane prefers the equatorial conformation with a Keq = 42 at

room temperature:

a. What percent of molecules are in the axial form at room temperature?

8. Draw a diagram that shows cyclohexane flipping from one chair, to a boat, to the

other chair conformation.


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Material Covered on the Unit 2 Exam

1. Be able to describe the process of fractional distillation

2. Be able to describe the occurrence and uses of the important alkanes covered in this

packet

3. Be able to use proper IUPAC formatting as well as common names to name

compounds

4. Be able to draw structures if given an IUPAC or common name

5. Be able to draw sawhorse and Newman projections down the bond axes of simple

alkanes.

6. Be able to draw and interpret an energy diagram.

7. Be able to describe and define all key terms.

8. Be able to complete a strain energy analysis

9. Be able to draw boats and chairs, paying attention to axial and equatorial positions.

Documents

Unit 2 Packet - Wikispaces2+Packet.pdf · Unit 2 Packet: Alkanes: Sources, Uses, Nomenclature ... Honors Organic Chemistry 37 Alkanes: Nomenclature Alkane names: n 1 Methane 2 Ethane