Saturated hydrocarbons Chapter 12. Organic and inorganic compounds Organic chemistry concerns the...

Saturated hydrocarbons

Chapter 12

Organic and inorganic compounds• Organic chemistry concerns the chemistry of

carbon compounds contain C and H, but also maybe other p-block elements.

• At the time of printing (Stoker, 5th edition), there were around 5 million organic compounds catalogued

• Inorganic chemistry concerns the chemistry of the other 117 elements. Around 1.5 million of these are known

A polar,charged group

Non-polar

hydrocarbon tail

Organic and inorganic compounds

• The reason why there are so many organic compounds is that carbon is very good at forming bonds with other carbon atoms.

• Carbon atoms are commonly found in chain-like arrangements or C-rings (or both within the same molecule).

• Carbon has four valence electrons. In organic compounds, it forms four covalent bonds to obtain an octet.

foursinglebonds

double bond,2 singlebonds

triple bond,single bond

two double bonds

C C C C

Hydrocarbons and hydrocarbon derivatives

• Hydrocarbons are compounds that contain only carbon and hydrogen in their formulas.

• Two basic categories of hydrocarbon:– Saturated hydrocarbons: all carbon atoms are

connected together with single bonds– Unsaturated hydrocarbons: involve one or more

multiple (double, triple) C-C bonds• Hydrocarbon derivatives contain carbon and

hydrogen, and one or more other elements (P, N, O, Cl, etc.)

Hydrocarbons and hydrocarbon derivatives

• Saturated hydrocarbons may be found in two possible formats:

an acyclic, 6-C chain

a cyclic 6-C structure

C

C

C

C

C

C

C CCCCC

Alkanes: acyclic saturated hydrocarbons

• An alkane is a saturated hydrocarbon that is acyclic (does not possess ring-structure).

• Because all C-C bonds are single bonds (and because the other bonds that carbon needs to get an octet are to H-toms), alkanes have the general formula CnH2n+2 (n = # of C-atoms)

Examples of alkanes:CH4

C2H6

C3H8


• In an alkane, each carbon is tetrahedral (it has four bonds to other atoms. Rem: VSEPR)

CH4 C2H6 C3H8


• Chemical formulas for alkanes are written as CnH2n+2; however, structural formulas give more information.– Chemical formula reveals the type and number of each

element in the compound– Structural formulas show how each atom in the molecule

is connected

expandedstructuralformula

condensedstructuralformula

name

CH4

methane

CH3-CH3

ethane

CH3-CH2-CH3

propane

C

H

H

HH C H

H

H

C

H

H

C

H

H

HC

H

C

H

H H

HH


• For longer carbon chains, an abbreviated, condensed structural formula is advantageous, as it shows most of the information of the expanded formula without taking up as much space

CH3-CH2-CH2-CH2-CH2-CH2-CH2-CH3 CH3-(CH2)6-CH3

6 –CH2- units betweentwo CH3-groups

An 8-carbon chain (two CH3-groups linked by a 6-carbon chain (6-CH2- units)


• Sometimes, a simple skeletal structural formula can be used to convey hydrocarbon structure

CH3-CH2-CH2-CH2-CH3

skeletal structural formula condensed structural formula

C CCCCmean

the samething

Alkane isomerism

• The types of alkanes we’ve considered so far involve “straight chain” types, where the carbon atoms form a continuous series (i.e. no branches).

• When alkanes having four or more carbons are considered, there is more than one structural formula that can be used to describe a given molecular formula.

Alkane isomerism

• The formula C4H10 can be represented by the following condensed structural formulas:

butane(or n-butane) isobutane

C H

C H 3

C H 3C H 3C H 2 C H 3C H 2C H 3

These compounds possess the same chemical formula,but differ in the way the atoms are arranged (isomers)

Alkane isomerism

• The C4H10 shown on the left is called a continuous chain alkane (or an unbranched/”straight-chain” alkane).

• The one on the right is called a “branched-chain” alkane

butane(or n-butane) isobutane

C H

C H 3

C H 3C H 3C H 2 C H 3C H 2C H 3

These are called “constitutional isomers” which differ in their atom-to-atom connectivity

Alkane isomerism• As the number of carbon atoms in the alkane

grows, so do the number of possible isomers.

pentane

isopentane neopentane

C H 2 C H 3C H 2C H 2C H 3 C

C H 3

C H 3

C H 3C H 3C H

C H 3

C H 3C H 2C H 3

Conformations of alkanes

• The carbon-carbon bonds in alkanes permit rotation of each carbon-group with respect to the others that are chemically bound to it.


• Conformations are specific, 3-dimensional arrangements of atoms in organic molecules (at some instant) that result from rotation about C-C single bonds.

• Several conformations of a six-carbon chain are shown using the skeletal structures below:

C CCCCC

C

C

CCC

C

C

C

CCC

C

C CCCC

C

All the same molecule: C6H14


• Note that the following two skeletal structures describe two different alkanes:

C

C

C CCCC CCCC

C

Alkane on the left is a 6-carbon, continuous chain structure.Alkane on the right is a branched structure (a 5-carbon, continuous

chain that has a 1-carbon branch)


• Do the following pairs of condensed structural formulas describe the same alkane?

a)

C H 2 C H 3

C H 2C H 2

C H 2C H 3

C H 2 C H 3C H 2C H 2C H 2C H 3

b)C H 2

C H 3

C H

C H 3

C H 3C H

C H 3

C H 2 C H 3C H 3

c) C H

C H 3

C H 2 C H 3C H 3 C H 2

C H 3

C H 2

C H 3

C H 2

IUPAC nomenclature for alkanes

• The names that have been shown for the branched alkanes so far are common names (made as these compounds were identified).

• As the number of organic compounds catalogued grew, a system for naming was developed by the International Union of Pure and Applied Chemistry (IUPAC).

• The basic system used is one that employs a prefix-type name.


• Names for continuous chain alkanes (first ten) are shown below. The names use a prefix (e.g. meth-) to designate the number of carbon atoms in the chain.

Meth-Eth-Prop-But-Pent-Hex-Hept-Oct-Non-Dec-

Prefix

Alkanes have ”ane” atthe end of their name


• Branched-chain alkanes can be described as continuous-chain alkanes with branches (substituents).

• The IUPAC system of naming branched-chain alkanes describes the type and location of substituents before the name of the longest, continuous chain of carbon atoms in the alkane.

Substituents are the “branches” in branched-chain alkanes. They are atoms (or groupsof atoms for the kind we’ll look at first) that hang off the main carbon chain.


• Substituents in branched-chain alkanes are called alkyl groups. An alkyl group is the group of atoms that would be created by removing a hydrogen atom from an alkane. They are named according to the alkane from which they are derived.

methane a "methyl" substituent

propane a "propyl" substituent

C

H

H

HC

H

H

HH

C

H

H

C

H

H

C H

H

H

H C

H

H

C

H

H

C

H

H

H

To get the substituent name:take the alkane name andreplace the “ane” part with “yl”


• To name a branched alkane, follow these steps:1) Identify the longest, continuous carbon chain in the structure. This will be the base of the branched alkane’s name.

a 4-carbon, continuous chain

C H

C H 3

C H 2 C H 3C H 3

So far, we know this compound is going to be called some kind of butane)


2) Number this chain in a way that gives the carbon(s) with the substituent the lowest possible, overall numbering.

The methyl substituent is thus located on C-2 (carbon-2)

1 2 3 4

C H

C H 3

C H 2 C H 3C H 3

(something something butane)


• After locating the alkyl substituent by number, prefix the parent-chain alkane name (the longest, continuous carbon chain) with the number and the name of the substituent:

1 2 3 4

C H

C H 3

C H 2 C H 3C H 3

2-Methylbutane

Separate the number from the substituent name with a hyphen, and the last substituent name reads directly into the parent chain alkane name


• Another example

C H 2

C H 3

C H 2C H 3

C H

C H 2

C H 2 C H 2

C H 2C H 2

C H 3

1) Find the longest, continuous chain of C-atoms2) Number them in a way that gives all substituents the lowest total numbering3) Prefix the name of the parent alkane with the number and name of the substituent


• One with multiple substituents:

CH

CH 3

CH

CH 3

CH 2 CH 3CH

CH 3

CH 3

In cases where multiple substituents of the same type are present, prefix thesubstituent name with di-, tri-, tetra-, etc. to indicate how many of them are present

2,3,4-Trimethylhexane


• If more than one kind of substituent is present, the alphabetic order of the substituents take priority over the number of the substituent when numbering the parent chain

C H 2 C H 3C H

C H 3

C H 2

C H 3

C H C H 2C H 3

3-Ethyl-2-methylhexane(not 2-Methyl-3-ethylhexane)

Separate differentsubstituents with hyphens

C H 2CH 3

C H 2

C H 3

C H 2

C H

CH 2

CH 3

CH 2

CH

C H 2

C H 3

C H CH 2 C H 3C H 2

3-Ethyl-4,5-dipropyloctane

The prefix part of the propyl substituentsare not counted for alphabetical ordering

(come back to this structure later)


• IUPAC punctuation rules:1. Separate numbers from letters with hyphens2. Separate numbers from other numbers with

commas3. Don’t separate the last substituent name from

the parent alkane chain

4-Ethyl-2,3-dimethyl-5-propylnonane

31

2

Line-angle structural formulas for alkanes

• Line-angle structural formulas describe carbon-carbon bonds with straight lines (each point in the diagram represents a carbon atom with four bonds to carbon(s) and hydrogen(s) around it)

= C H 2 C H 3C H 2C H 2C H

C H 3

C H 3

= C H2 C H3C H2C H3

It is understood that each C-atom has four bonds; C-H bonds are there, but not shown

Classification of carbon atoms

• The carbon atoms in organic structures are classified as primary, secondary, tertiary, or quaternary, depending on the number of other carbon atoms bound to them.– Primary (1o) C: bounds to one other C-atom– Secondary (2o) C: bound to two other C-atoms– Tertiary (3o) C: bound to three other C-atoms– Quaternary (4o) C: bound to four other C-atoms

C

CH 3

CH 3

CH

CH 3

CH 3CH 2CH 3

1o

2o

3o4o

Branched-chain alkyl groups

• Sometimes, branched-chain substituents are encountered. These are named according to the parent alkane from which they are derived.

C H 2C H 3

C

C H 3

C H 3C H 3

C H C H 2 C H 3C H 2C H 2C H 2C H 2C H 3

CH C H 3C H 3

CH C H 2 C H 3C H 2C H 2C H 2

4-Isopropyloctane 4-tert-Butyloctane

Substituent derives from a 3-C alkane(propane) and point of attachment

is a secondary C of the alkane

Substituent derives from a 4-C alkane(butane) and point of attachment

is a tertiary C of the alkane

could also call this 4-sec-Propyloctane


…another point: for the purposes of capitalization (at the beginning of the name), tert- andsec-are not capitalized, but iso is

C H 2C H 3

C

C H 3

C H 3C H 3

C H C H 2 C H 3C H 2C H 2C H 2C H 2C H 3

CH C H 3C H 3

CH C H 2 C H 3C H 2C H 2C H 2

4-Isopropyloctane 4-tert-Butyloctane


• Given a choice between unbranched substituents and branched substituents, use unbranched ones for naming

C H 2 C H 3C H

C H 3

C H 2

C H 3

C H C H 2C H 3

3-Ethyl-2-methylhexane(not 2-Methyl-3-ethylhexane)also, not 3-Isopropylhexane

Cycloalkanes

• Cyclic alkanes (cycloalkanes) are alkane chains where the end carbons are linked together (need to kick off 2 H atoms from the formula of the corresponding straight-chain alkane to get the cycloalkane formula).

• The general formula for a cycloalkane is CnH2n

Cyclopropane

Cyclobutane

Cyclopentane

Cyclohexane

Cycloheptane

Cyclooctane

Cyclononane

=

C6H12 C6H12

C H 2

C H 2

C H 2

C H 2

C H 2

C H 2

Cycloalkanes

IUPAC nomenclature for substituted cycloalkanes

• If one substituent exists on a cycloalkane, no numbering is needed to denote its location

CH 2

CH 3CH

CH 2

CH 2

CH 2

CH 2

CH 2

Ethylcyclohexane

IUPAC nomenclature for substituted cycloalkanes

• If two substituents are present, the ring is numbered follows alphabetic priority.

• If more than two substituents are present, the ring numbering is assigned in a way that gives the lowest overall substituent numbers (order they are reported in is still alphabetic)

1-Ethyl-2-methylcyclohexane2-Ethyl-1-methyl-4-propylcyclohexane(not 1-Ethyl-2-methyl-5-propylcyclohexaneor 1-Methyl-2-ethyl-4-propylcyclohexane)

1

2

3

4

5

6

C H

C H 3

C H 2

C H 3C H

C H 2

C H 2

C H 2

C H 2

1

2

3

4

5

6

C H 2C H 3

C H 2

C H

C H 2

C H 3C H

C H

C H 3

C H 2

C H 2

C H 2

Isomerism in cycloalkanes

• Constitutional isomers are possible for cycloalkanes having four or more carbons:

C4H8

Cyclobutane Methylcyclopropane

C4H8

These isomers differ in the way the carbon atomsare connected together (constitutional isomers)


• As before, as the number of carbons in the (cyclo)alkane grows, so do the number of constitutional isomers.

C5H10

Cyclopentane Methylcyclobutane 1,2-Dimethylcyclopropane Ethylcyclopropane

C5H10 C5H10 C5H10

Isomerism in cycloalkanes• Another kind of isomerism we haven’t yet encountered, called

stereoisomerism, involves molecules that have the same molecular formula, same atom-to-atom connectivity, but differ in the 3-dimensional arrangement of the atoms in space.

• In cycoalkanes, there may exist the possibility of cis-, trans- isomers

CH3-substituents areboth above the C-Cbond of cyclopentane

cis-1,2-Dimethylcyclopentane trans-1,2-Dimethylcyclopentane

One CH3-substituent isabove the C-C bond ofcyclopentane and theother one is below it

C H 3

H

H

C H 3C

C C

C

C

H

C H 3

H

C H 3C

C C

C

C


• There are two distinct molecules. One can’t be converted into the other without breaking bonds first.

• Can have this form of isomerism for any cycloalkane that has more than one substituent.

cis-1-Ethyl-2-methylcyclohexanetrans-1-Ethyl-2-methylcyclohexane

H

C H 3

HC

C C

C

CC

C H 3

H

H

C

C C

C

CC

C2H5

C2H5


• Substituents also don’t need to be on adjacent carbon atoms of the ring (but can’t be on the same carbon atom of the ring)

trans-1-Ethyl-3-methylcyclohexane cis-1-Ethyl-3-methylcyclohexane

H

H

C H 3

C

C C

C

CC

C H 3

H

HC

C C

C

CC

C2H5 C2H5

Sources of alkanes and cycloalkanes

• The crude petroleum that is obtained at drilling sites is a mixture of hydrocarbons (cyclic and acyclic) that is purified (refined) by taking advantage of the boiling point differences of the various components

Sources of alkanes and cycloalkanes

• Boiling point is observed to increase with increasing chain C-chain length (and ring size for cycloalkanes).

• About a 30o increase per additional C (–CH2- unit) in the chain.

Physical properties of alkanes and cycloalkanes

1. Alkanes and cycloalkanes are water-insoluble

2. Alkanes and cycloalkanes have densities that are less than that of water (0.6 – 0.8 g/mL, as compared to ~1 g/mL for H2O)

3. Boiling points of continuous chain alkanes and cycloalkanes increase with an increase in carbon-chain length or ring size

• Cycloalkanes have higher boiling points than corresponding alkanes because they are more rigid• Branched chain alkanes have lower boiling points because they are more compact and have less surface areas that straight-chain forms

Chemical properties of alkanes and cycloalkanes

• Alkanes and cycloalkanes have low chemical reactivities. The C-C bonds and C-H bonds are non-polar, which do not encourage reactions with other species, and the bond strengths are fairly high (strong bonds)

• Two reactions that they are susceptible to are combustion and halogenation


• In a combustion reaction, alkanes and cycloalkanes are reacted with O2 to form CO2 in an oxygen-rich environments (or CO or other C-products in less O2-rich environments).

• Some examples of alkane combustion reactions:

CH4 + 2O2 CO2 + 2H2O + heat

2C6H14 + 19O2 12CO2 + 14H2O + heat


• Halogenation reactions involve halogen atoms (group 7: F, Cl, Br, I). These reactions involve the substitution of a hydrogen atom of an alkane with a halogen:

Br2 HBrheat or light

BrH

H

H

H

H

CCHH

H

H

H

H

CC + +

R-H X2 R-X H-Xheat or light

+ +

In general, the reaction goes like this:


• In a substitution reaction, an atom or group of atoms is replaced by another atom (or group of atoms) in a hydrocarbon or hydrocarbon derivative.

• On the surface, they are like the replacement reactions we’ve already studied


• In situations where more than one type of product can result, a mixture of products is usually obtained.

• In many cases, more than one H-atom can be substituted:

Br2

Br2

Br2

Br2

HBr

HBr

HBr

HBr

heat or light

heat or light

heat or light

heat or light

H C

H

H

Br C

Br

H

H Br

H C

Br

H

Br C

Br

Br

H Br

H C

Br

Br

Br

H C

H

H

H C

H

H

H Br

C

Br

Br

Br Br

+

+

+

+

+

+

+ +

B r

2

Br2

Br2

Br2

HBr

HBr

HBr

HBr

heat or light

heat or light

heat or light

heat or light

H C

H

H

Br C

Br

H

H Br

H C

Br

H

Br C

Br

Br

H Br

H C

Br

Br

Br

H C

H

H

H C

H

H

H Br

C

Br

Br

Br Br

+

+

+

+

+

+

+ +

Nomenclature and properties of halogenated alkanes

• Halogenated alkanes (or haloalkanes) are hydrocarbons (or their derivates) that possess at least one halogen atoms

• Naming rules– Halogens are treated just like other (alkyl) substituents

when numbering and alphabetic naming are considered– Substituents are called fluoro-, chloro, bromo-, and iodo-

for the purposes of assigning names

2-Chloro-3-methylbutane 3-Bromo-1-chlorobutane

1-Ethyl-2-fluorocyclohexane

C H

F

C H 2

C H 3C H

C H 2

C H 2

C H 2

C H 2

C H 2

Cl

C H

Br

C H 2C H 3C H

Cl

C H

C H 3

C H 3C H 3

Nomenclature and properties of halogenated alkanes

• In terms of chemical reactivity, halogenated alkanes are more reactive than alkanes and cycloalkane analogues, because the C-X bond (X = halogen) makes the bond polar and thus susceptible to reactions that require initial dipole-dipole interactions.

• Some halogenated alkanes have densities greater than that of water.– Chloroalkanes with 2 or more Cl-atoms– Bromoalkanes– Iodoalkanes

Saturated hydrocarbons Chapter 12. Organic and inorganic compounds Organic chemistry concerns the...

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