Ch. 12 - Alkanes (sat’d HCs)

Alkanes

Acyclic Saturated Hydrocarbons (chains)

General Formula: CnH2n+2

Structural: (ex.: C4H10)

Complete

Condensed

Skeletal

Line AngleDecomposition of plant and animal matter in marshes is a good source of methane gas.

Alkane Nomenclature

MethaneEthanePropaneButanePentaneHexaneHeptaneOctaneNonaneDecane

Alkane Isomerism

Isomers: compounds that share the same molecular formula but have different structural formulas

With an increased number of C atoms, there is an exponentially increased number of isomersConstitutional - same molecular formula, different structural formula;

differ in connectivity of atoms

Ex. C4H10 butane ; isobutane

Ex. C8H18 octane; 3-methyl heptane

Alkyl Groups & IUPAC namesUse the following rules to properly name hydrocarbon molecules:

1. Identify the longest, continuous chain of C atoms and name it (parent C chain and suffix.2. # the C atoms in the chain from the end nearest an alkyl group.3. # and name the attached alkyl group(s).4. If more than one alkyl group:

1st substituent must have lower number.Alkyl groups are listed in alphabetical order.If two or more identical substituents are bonded, use prefixes to indicate how many

5. Separate #s from each other with commas; separate names from numbers with hyphens; do not use a hyphen/space after the last substituent, before the parent alkane name.

Carbons are classified 4 ways in a chain:Primary, secondary, tertiary, quaternaryBased on the # of C atoms to which the carbon atom is bonded.

Branched-chain Alkyl Groups

“Simple”: 4 important ones to know!

Complex: “select the longest chain as the “base” alkyl, add “substituents.”

Ex.

Cycloalkanes

Isomerism of Cycloalkanes

Constitutional (ex.: C5H10) 5 isomers

Stereoisomers: possible with substituted cycloalkanes (ex.: 1,2-Dimethylcyclohexane)

Cis- (SAME side)

Trans- (ACROSS from)

Sources of Saturated Hydrocarbons

Natural Gas:Methane (50-90%)

Ethane (1-10%)

Propane & Butane (up to 8%)

Petroleum (crude oil): this is a complex mixture of both cyclic and acyclic hydrocarbons which can be separated by a process known as fractional distillation.

Physical Properties of Hydrocarbons

All are insoluble in waterConsider the polarity of the compoundsTherefore they can make good protective coatings

All are less dense than water (0.6-0.8 g/mL)

Oil & water

Boiling Points & States of MatterGenerally, BP increases with increasing # C atoms

Reason: increasing LDF

1-4 C atoms = gas; 5-17 C atoms = liquid; >17 C atoms = solidIsomers: Branched BP < Unbranched BPCyclic Compounds have higher BP than Acyclic.

Chemical Properties of Hydrocarbons

Alkanes are the least reactive organic compounds (they have no fcn’l groups). However, two major reactions are common:

CombustionR (some hydrocarbon) + O2 → CO2 + H2O + energyenergyvery exothermic reaction.

If the quantity of O2 is insufficient, it will form a poison called carbon monoxide (CO). Here is an example with methane:

CH4 + 2 O2 → CO2 + 2 H2O

with less O2:

2 CH4 + 3 O2 → 2 CO + 4 H2O (poison!)

with even less O2:

CH4 + O2 → C + 2 H2O (black soot forms)

HalogenationR-H + X2 --> R-X + H-X

Hydrocarbon fuelsFossil fuels (solid, liquid, or gas) form from organic material being covered by successive layers of sediment over millions of years

Oil & natural gas: slow decomposition & burying of marine phytoplankton & zooplankton that sank to the sea floor.  Coal: ancient swamps and bogs - slow decomposition of land plants in anaerobic conditions:

e.g. peat bogs of Ireland

Petroleum, Naphtha, or crude oilFrom Greek petra = rock and oleum = oil

Thick, dark brown or greenish liquid

Complex mixture of various hydrocarbons,

largely of the alkane series

May vary much in appearance, composition, and

purity

Petroleum is also the raw material for many chemical products: fuels & solvents

Can be altered into: fertilizers, pesticides, and plastics

Think of the impact of a substantial oil crisis on our economy & society!

A rock formation such as this is necessary for the accumulation of petroleum and natural gas.

Crude Oil History•1st oil wells drilled in China in 4th century depth up to 800 ft. •Drilled using bits attached to bamboo poles •Burned oil to evaporate brine & produce salt.•10th century, bamboo pipelines connected oil wells with salt springs.

Ancient Persian tablets indicate the medicinal and lightinglighting uses of petroleum in the upper society.8th century: streets of Baghdad paved with tar.In the 9th century, oil fields were exploited in Baku, Azerbaijan, to produce naphtha.

Discovery of oil in 1859, near Titusville, Pennsylvania. Great demand for kerosene and oil lamps. Medicinal purposes, lighting, and lubricants for new steam engines.Introduction of internal combustion engine lead tomajor "oil booms" in Texas, Oklahoma, and California.

The complex hydrocarbon mixture present in petroleum is separated into simpler mixtures by means of a fractionating column.

Differences in BPs of hydrocarbon chains

can separate out sections of

the crude oil:Fractional Distillation

Fuel type    Mega Joules / Liter 

BTU / US gal    Research octane #

Gasoline 29.0 125,000 91 to 98

Diesel fuel oil

32.19 138,690 N/A

Residential heating oil

34.74 149,690 N/A

Gasahol10% ethanol

90% gasoline

28.06 120,900 93 to 94

Octane: major component of gasoline

•Octane is an alkane hydrocarbon:CH3(CH2)6CH3. •octane isomer: 2,2,4-Trimethylpentane = 100 pts on the octane rating scale

•n-heptane is the zero point (greatest engine knocking or “pinging”)•Octane ratings are used to represent antiknock performance (less premature combustion)

Natural gasNatural gas mainly methane (does contain other “small” HC cpds).Highly flammableNo ash and very little air pollution. From light portion of petroleumRises thru fissures in earth’s crustMan-made wells can tap it Discovered thousands of years ago could be burned for heat and light. Colorless, odorless, & lighter than air.Mercaptan, chemical odorant, is added for gas leaks. natural gas combustion CO2 + H2ONatural gas found in different underground formations: shale, sandstone beds, coal seams, & deep, salt water aquifers

New sources of methane gas:Decomposition of organic

matter in landfills

CH4 gas can be tapped instead of vented

Kivu Lake

3 lakes contain large concentrationsof dissolved CO2 gas.

Lake Nyos & Lake Monoun sites of gas explosions: 40 dead at Monoun & 1800 at Nyos

3rd lake is Kivu, which contains a 1,000x more gas than Lake Nyos.

Coal formation

•Vegetable matter accrues •Prevented from decay•Forms peat beds. •Over time: buried & compressed, forms lignite.

Increased P & T makes bituminous

coal (higher C content). •At great depths, high temps reduce CH4 & forms anthracite (very high in C)

Fossil Fuel “Facts”

                                         

Source: Shell Oil

Current Consumption?What about as China and India

modernize?These countries

have 33% of world population!

The U.S. has 4% of the World's population. The U.S. uses 34% of Earth's natural resources.

U.S. Sources of Energy Production

Fossil Fuels 86% Geothermal 0.5% Nuclear 8% Wind Farms 0.1% Hydroelectric 2% Solar 0.1% Biofuels 3.3%source: US Dept of Energy

Remember that, while hydrocarbons serve as a transportation fuel, they are also used to produce plastics, etc.

Enormous dependence on a limited resource.Other alternatives?

-Wave energy-Tidal energy-New technology for solar panels, turbines, etc.

-Conservation!!-Efficiency!!!

Problems associated with alkane hydrocarbons

Hydrocarbon pollution: (oil spills) of aquatic environments (e.g. BP in the Gulf of Mexico)

Global warming: CO2 and H2O

Acid rain due to sulfur impurities in oil and coal: damage crops, lakes, buildings, etc.

Smog and soot: increase respiratory problems

Land fill: non-decomposing plastics

Ozone depletion / increased UV radiation

Accidental & Purposeful Oil Spills

Clean-up takes into account density and non-polar nature

Global warming: Global warming: The The Greenhouse effect:

Short wavelength solar radiation releases energy

as it hits molecules.Turns into long

wavelength energy.Gases in atmosphere trap it and warm the

atmosphere.

Arctic ice sheet 1979 and 2000

CO2 increase correlates to average global temperature increase

Can cause: changes in sea level, reduction of reflective ice caps, increased storm ferocity, plant & animal re-distribution,shift ocean currents, createdroughts & forest fires, increased short-term temperature fluctuations.

Alkane substitution reaction:Incoming atom or group of atoms (orange sphere) replaces a hydrogen atom in the alkane model.

Naming:Treat halogen atoms like alkyl groups.

F = fluoro; Cl = chloro; Br = bromo; I = iodo

Ex.: CH3-CHBr-CHBr-CHI-CH2-CH3

Halogenation Reactions

General equation: RH + X2 → RX + HX

Hydrocarbon + Halogen Halogenated + acid (diatomic) hydrocarbon

Ex. CH4 + Cl2 --> CH3Cl + HCl

Highly exothermic reaction: can lead to an explosion

The process can continue to alter the resulting products as long as the halogen remains in sufficient quantities to drive further reactions. (The halogen would be the __________ reactant.)

Space-filling models of the four ethyl halides.

Do these molecules act as polar or non-polar?

Chlorofluorocarbons (CFCs)

Developed in the 1930's

Very stable compounds composed of C, F, Cl, & H

Freon is the tradename:

Trichlorofluoromethane

Dichlorodifluoromethane

Trichloro-trifluoroethane Dichloro-tetrfluoroethane Chloropentafluoroethane

Safe, non-toxic,

non-flammable

alternative to

dangerous

substances

(e.g. ammonia)

for aerosol-spray

propellants,

refrigerants,

solvents, and

foam-blowing agents

CFCs and refrigeration

CFCs and propellants

UV radiation in the stratosphere

The Ozone Layer Chemistry

CFCl3 + UV LightUV Light ==> CFCl2 + Cl Cl + O3 ==> ClO + O2

ClO + O ==> Cl + O2

The chlorine free radical atomis then able to attack anotherozone molecule

Cl + O3 ==> ClO + O2

ClO + O ==> Cl + O2

and again ... Cl + O3 ==> ClO + O2

ClO + O ==> Cl + O2

and again... thousands of times!A catalyst!

The ozone destruction process requires conditions cold enough (-80oC) for stratospheric clouds to form. Once these stratospheric clouds form the process can take place, even in warmer conditions

Ozone consumption has been greatly reduced,

1997 ozone

hole

2003 ozone

hole

however CFCs may linger for another 150 years in the atmosphere

Ozone layer thickness

What do you need to know (about saturated hydrocarbons)?

Structural characteristics (know the functional group)Substituents

Nomenclature (the rules for naming the molecules)

Physical and Chemical properties (basic/simple)

Occurrence and uses (common)

Preparation (what basic reactions produce the molecules)

Characteristic reactions of the molecules