Download - HyrdoCarbon Notes Chemistry 11 2011 2003

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Hydro Carbon Organic Chemistry

Organic Chemistry is the study of covalent compounds which contain carbon.

ex: CH4, C8H18, C6H12O6, CCl2F2

There are more carbon-containing compounds than all other types of compounds put together,

making up more than half of all known compounds. Why carbon?

Carbon is able to bond with other carbon atoms, to form chains, rings, spheres and sheetsof almost any size (no other element is so versatile).

Carbons tetravalence (four bonding sites) enable it to form branched chains, and single,

double or triple bonds.

Organic compounds do not include:

most common ionic compounds containing carbon (ex: CO32-, C2H3O2

-, CN-)

oxides of carbon (CO, CO2)

I. Hydrocarbons

Compounds containing only C and H 2 types of hydrocarbons:

(1) aliphatic hydrocarbons do not contain benzene rings

(2) aromatic hydrocarbons contain benzene rings or similar structures

saturated vs. unsaturated hydrocarbons

saturated hydrocarbons contain only single bonds (no double or triple bonds)

unsaturated hydrocarbons have one or more double or triple bonds

homologous series a set of compounds whose nearest neighbors differ by one repeating

unit, most often a methylene group ( CH2 )

straight-chain alkanes: CnH2n+2

straight-chain alkenes: CnH2n straight-chain alkynes: CnH2n-2

Members of a homologous series have similar chemical properties (they undergo similar

reactions), and have a gradation in physical properties (such as melting and boiling point.).

The boiling points of alkanes gradually increases as the number of carbons increases.

Plotting the boiling point against chain length (# of carbon atoms) gives a steep graph at

first, but flattens later (refer to attached graph/data table).

In other words, the bigger the molecule, the higher the melting and boiling points.

Hydrocarbons are effectively non-polar because of the symmetry of the molecules and

the very low polarity of the C H bond (an electronegativity difference of 0.4 means it is

effectively a non-polar bond).

Thus, the only intermolecular forces acting on hydrocarbons are London Dispersionforces.

Larger molecules have more surface area, so more attractive forces exist between

molecules, requiring more energy to separate them, resulting in higher boiling points.

In general, chain-branching causes a slight decrease in boiling point, compared to the

straight-chain compound with the same number of carbons. This is because the molecule


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becomes more compact, with a smaller surface area, reducing the intermolecular

attractions.

I. a. Alkanes(also calledparaffins)

hydrocarbons containing onlysingle bonds (they are all saturated)

alkanes have the general formula CnH2n+2

name formula structural formula structural isomers

methane CH4

ethane C2H6

propane C3H8

butane C4H10

pentane C5H12

hexane C6H14

heptane C7H16

octane C8H18

nonane C9H20


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decane C10H22

all alkanes end in -ane

structural isomers compounds with the same molecular formula, but different structural

formula (i.e. same # of Cs and Hs but theyre arranged differently)

examples:

Pentane 2-Methylbutane

Naming alkanes Rules for nomenclature:

1) Find the longest continuous (though not necessarily straight) chain of carbon atomsYou name the hydrocarbon based on this longest chain.

2) Any carbon chain branching off from the longest chain is named as alkyl group

Alkyl group use the same prefixes, replacing ane with yl

3) Each carbon must have a number. Carbons are numbered in the direction giving the

lowest possible number for all alkyl groups (substituents).

4) When there is more than one of thesame alkyl group, use prefixes like di-, tri-, tetra,

penta-, etc. and identify the # of the carbon(s) on which they are found

Use the lowest possible numbers!

Position #s are separated by commas, and are followed by a hyphen.

5) When there are 2 are more differentalkyl groups, put them in alphabetical order precededby their position number.

methylbutane 2,3-dimethlypentane 3-ethyl-2-methylpentane

(2-methylbutane)

(you dont need to use a number prefix

if no other position is possible)

Alkanes are relatively unreactive due to the inertness of the C-H and C-C bonds.

The inertness of the C-H and C-C bonds is due to 2 main factors:

1. low bond polarity there is little to no difference in electronegativity between the

bonded atoms (C = 2.5, H = 2.1 effectively a non-polar covalent bond)

2,2-Dimethylpropane


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2. high bond energy these bonds are very short and thus very difficult to break

Alkane m.pt. / C b.pt. / C

CH4 -183 -162

C2H6 -172 -88C3H8 -188 -42

C4H10 -138 0

C5H12 -130 36

C6H14 -95 69

C7H16 -91 98

C8H18 -57 126

C9H20 -54 151

C10H22 -30 174

C11H24 -26 196

C12H26 -10 216

C13H28 -5 235

C14H30 6 254

C15H32 10 271

C16H34 18 287

C17H36 22 302

C18H38 28 317

C19H40 32 331

C20H42 36 344

Figure 2.1: Melting and Boiling Points of Straight-Chain Alkanes

Figure 2 .2: Bo iling Points of Straight-Chain Alkane

-200

-100

0

100

200

300

400

0 4 8 12 16 20

Number of Carbon Atom

Temperature/deg.

C


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Isomers compounds with the same molecular formula but differentstructural formula

Draw and name all the isomers with a molecular formula C5H12

Alkyl group (Side Groups) a hydrocarbon group that is attached tothe parent chain of a molecule

Alkyl groups to know and love!!

Methyl Ethyl

Propyl (n-propyl) isopropyl

Butyl (n-butyl) s-butyl (secondary butyl)

Isobutyl t-butyl (tertiary butyl)

Cycloalkanes cycloalkanes alkanes in which the carbon atoms form a ring

The general formula for cycloalkanes is CnH2n

examples:


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cyclopropane cyclobutane cyclopentane cyclohexane

naming cycloalkanes:

1) Use the prefix cyclo- before the number of carbons in the ring.

2) Single substituents do not need to be numbered, since all positions are equivalent.

3) When there is more than one of thesame functional group, use prefixes like di-, tri-, tetra,penta-, etc. and identify the # of the carbon(s) on which they are found.

Use the lowest possible numbers!

Position #s are separated by commas, and are followed by a hyphen.

4) When there are 2 or more differentalkyl groups, put them in alphabetical order precededby their position number.

examples:

1,1-dimethylcyclopentane 1-ethlymethylcyclobutane propylcyclohexane

I. b. Alkenes(also called olefins)

hydrocarbons which contain at least one double bond

alkenes have the general formula CnH2n

examples:

name formula structural formula

ethene C2H4

propene C3H6

(you dont need to use a number

prefix if no other position is possible)


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butene C4H8

(1-butene)

2-butene C4H8

2-methylpropene C4H8

all alkenes end in -ene

Naming alkenes

When naming alkenes, find the longest chain containing a double bond.

Carbon atoms of the longest chain are numbered from the end closest to the double bond.(i.e. assign each carbon in the longest chain a number, such that the double bond occurs

on the carbon with the lowest possible number)

The position number of the double bond is assigned to the first carbon atom of the double

bond (i.e. the lowest possible number carbon).

ex 1:

2-pentene


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ex 2:

3-heptene

If there is more than one double bond, use prefixes (di-, tri-, etc.) immediately before the

ene to indicate the number of double bonds, and use numbers separated by commas to

indicate their positions (again, number the carbons so that the first double bond is at the

lowest possible position).

ex 1:

1,4-hexadiene

ex 2:

Rules for naming alkyl groups attached to alkenes are similar to the rules for alkanes, with

the following difference:

The double bond takes precedence! Number the carbons so that the double bond occurs

at the lowest possible number carbon. Position numbers for alkyl groups thus depend onthe number assigned to the double bond.

Bond length and bond energy are inversely related.

The shorter the bond length, the higher the bond energy:

bond bond length (pm) bond energy (kJ/mol)

CC 154 347

C=C 134 611

C C 121 837

CH 110 414CN 147 293

CCl 176 330CO 143 351

C=O 120 745

(both from a carboxylic acid)

NN 140 159

OO 148 494

ClCl 199 240


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As the number of bonds increases, bond length decreases and bond energy increases

i.e. the C C triple bond has three shared bonds, and thus has a shorter bond length and

higher bond energy than the C=C and CC bonds.

However multiple bonds also tend to be much more reactive! As will be discussed later, it

is very common to break multiple bonds of alkenes or alkynes, using a variety of reagents to

form other products with single bonds.

Geometric Isomers

geometric isomers isomers in which the attached groups differ from each other relative to

the double bond

cis- isomers attached groups are on the same side of the double bond

trans- isomers attached groups are on opposite sides of the double bond

examples:

C4H8 C4H8

cis-2-butene trans-2-butene

I. c. Alkynes

Hydrocarbons which contain at least one triple bond

Alkynes have the general formula CnH2n-2

examples:

C2H2 C3H4 C4H6

ethyne propyne 1-butyne

(acetylene)


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All alkynes end in -yne

When naming alkynes, find the longest chain containing a triple bond.

Carbon atoms of the longest chain are numbered from the end closest to the triple bond.

The position number of the triple bond is assigned to the first carbon atom of the triple

bond (i.e. the lowest possible number carbon).

ex 1: see 1-butyne above

ex 2:

2-hexyne

The simplest member of the alkyne family (C2H2, or HC CH ) is also known commonly

as acetylene.

Acetylene is a gas commonly used in acetylene torches it burns with a very hot flame,

and is often used to cut steel.

Alkyne nomenclature also sometimes uses acetylene as the parent group, naming

functional groups coming off the acetylene (though this is not correct IUPAC

nomenclature)

ex: CH3CH2C CCH3

ethylmethylacetylene

Rules for naming functional groups attached to alkynes are the same as the rules for alkenes:

The triple bond takes precedence! Number the carbons so that the triple bond occurs at

the lowest possible number carbon. Position numbers for functional groups thus depend

on the number assigned to the triple bond.

II. Functional Groups

functional group an atom or group of atoms that undergoes characteristic, predictable

reactions

chemical reactions usually occur at functional groups!!!

Characteristic IR

Functional Group Name Absorption (cm-1) Example

alkenyl 1680 1620

CH3CHCHCH3

2-butene


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alkynyl 2260 2100

CH3CCCH3

2-butyne

halogenoalkane

(organic halide) CH3CH2Br

bromoethane

alcohol 3550 3200 (hydroxyl) CH3CH2OH

ethanol

(ethyl alcohol)

ether 1050 1200

CH3CH2OCH2CH3

diethyl ether

aldehyde 1740 1690

CH3CHO

RCHO ethanal

ketone 1750 1680

CH3CH2COCH3

RCOR' 2-butanone

carboxylic acid 1780 1710

(carboxyl)

CH3CH2COOH

RCOOH propanoic acidester 1750 1735

CH3COOCH2CH3

RCOOR' ethyl ethanoate

amine 3500 3300

CH3CH2NH2

RNH2, RNHR' ethylamine

amide NH (3700 3500)

C=O (1690 1630)CH3CONH2

RCONH2, RCONHR' ethanamide


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Functional groups can exist as isomers, so be careful!

ex: propanal (CH3CH2CHO) vs. propanone (CH3COCH3)

ethanol (CH3CH2OH) vs. dimethyl ether (CH3OCH3)