Chapt 21 Hydrocarbons [Selected]

21.1 Introduction to Hydrocarbons

21.2 Alkanes [Straight-Chain Only]

21.3 Alkenes & Alkynes (added)

21.4 Hydrocarbon Isomers [also with O]

Section 21.1 Introduction to Hydrocarbons

• Explain the terms organic compound and organic chemistry.• Identify hydrocarbons• Recognize the different ways that hydrocarbon molecules

may be represented (molecular formula, structural formula, ball-and-stick model, etc.) and convert a molecular formula into a valid structural formula and vice versa,

• Distinguish between saturated and unsaturated hydrocarbons.

Hydrocarbons are carbon-containing organic compounds that provide a source of energy and raw materials.

Section 21.1 Introduction to Hydrocarbons

Key Concepts

• Organic compounds contain the element ??

• Hydrocarbons are organic substances composed of carbon and ??.

• The major sources of hydrocarbons are petroleum and natural gas.

Organic Compounds

Name used because living organisms known to contain/produce them

Term applied to all carbon-containing compounds except for a small number of compounds considered to be inorganic

Because huge number exist, an entire branch of chemistry – organic chemistry – devoted to their study

Essential Organic ChemistryBond – force that hold atoms together in compounds

Carbon atom (C) always forms 4 bonds with other atoms; bond represented by a line

Hydrogen can only form one bond

Organic Compounds - Hydrocarbons

Simplest organic compounds; consist of only the elements carbon (C) and hydrogen (H)

In hydrocarbon, C either attached (bonded) to another C or to a hydrogen

Are thousands of hydrocarbons; can be in chain, branched chain, ring, and cage-like structures

Major source of hydrocarbons – petroleum and natural gas (mostly CH4 – methane)

HydrocarbonsCarbon atoms bond to each other by single, double, & triple bonds (always 4 total bonds)

Saturated hydrocarbons contain only single bonds

Unsaturated hydrocarbons contain at least one double or triple bond

Single DoubleTriple

Bond to some unspecified

atom

Simplest Hydrocarbon - MethaneChemical (molecular) formula: CH4

Structural formula:

C

H

H

H

HChemical bond

Carbon atom with 4 bonds

Simple Hydrocarbons - Methane

One carbon atom attached to 4 hydrogensShape of carbon bonded to 4 other atoms is a tetrahedron – bond angles of 109.5Hydrogens occupy corners of tetrahedron

109.5

Ways of Representing CompoundsCompounds may be represented by various types of formulas and graphical presentations

Variety of these shown on following slides

Chemists use form that best shows information they wish to highlight

Molecular formula most compact but no information about connections & geometry

Structural show connections but no 3D info

Most graphical forms can be generated and/or manipulated using online chemical software

Ways of Representing Methane

Chemical (molecular) formula CH4

C

H

H

H

H

Structural formula

Structural formula with additional geometry information – solid wedge coming toward you, dashed one away

Ball & stick

model

Space-filling model

Chapt 21 Hydrocarbons [Selected]

21.1 Introduction to Hydrocarbons

21.2 Alkanes

21.4 Hydrocarbon Isomers [also with O]

Section 21.2 Alkanes

• Name a straight-chain alkane from its molecular formula or by examining its structure (up to octane).

• Draw the structural formula or write the molecular formula of a straight-chain alkane when given its name (up to octane).

Alkanes are hydrocarbons that contain only single bonds.

Section 21.2 Alkanes

Key Concepts

• Alkanes contain only single bonds between carbon atoms.

• Alkanes and other organic compounds are best represented by structural formulas and can be named using systematic rules determined by the International Union of Pure and Applied Chemistry (IUPAC).

• Alkanes that contain hydrocarbon rings are called cyclic alkanes.

AlkanesSimple Alkanes – hydrocarbons with only single bonds and no ring structures

All have formula CnH2n+2 n = integer

All have names ending in “ane”

Simplest = methane

Chemical (molecular) formula: CH4

Structural formula: C

H

H

H

HChemical bond

Carbon atom with 4 bonds

Alkanesn = 2 ethane

Chemical (molecular) formula: C2H6

Structural formula:

n = 3 propane

Chemical (molecular) formula: C3H8

Structural formula:

C

H

H

H

C

H

H

H

C

H

H

HC

H

H

H

C

H

H

“Constructing” Alkanes Stepwise

Can think of alkanes larger than methane as being built from smaller molecules by adding a methyl group: CH3

Process: 1) Remove H atom (leave bond)

2) Replace removed atom with CH3

If start with CH4, four possible choices for H to remove, but all choices result in exactly the same molecule, ethane = C2H6

methaneCH4

H C

H

H

H

Making Ethane (C2H6) From CH4

C H

H

H

methyl group: CH3

ethaneC2H6

Ethane

Can write formula as C2H6 or as CH3CH3

Latter method allows one to visualize and draw structure more easilyAdditional very compact representation possible –skeletal (aka line-angle or bond line) formula Bonds are lines (as before)Carbon atoms present where line begins or ends or where 2 lines meetH not shown unless attached to drawn atom

As represented by skeletal formula

Ethane

Most alkanes rotate freely about the single bond between carbon atoms

Single Bond Free RotationFree rotation occurs about single bonds

Consequence of free rotation: 2 molecules that may appear different when drawn may in fact be identical because one molecule may be twisted about its single bonds to have the exact same shape as the 2nd molecule

Two Equivalent Butane Molecules

Right hand structure is twisted version of left hand structure

Single Bond Free RotationFree rotation occurs about single bonds

Because of free rotation all six hydrogen atoms in ethane are equivalent

If making new compound from ethane by replacing a hydrogen, doesn’t matter which one is chosen – result will be the same

H C

H

H

C H

H

HC H

H

H

Making Propane (C3H8) From Ethane

Ethane: C2H6

methyl group: CH3

Propane: C3H8 or CH3CH2CH3

As represented by skeletal formula

Note: “straight” chain shown in structural

formula isn’t

Straight-Chain Alkanesn = 3 propane

Propane: Molecular formula: C3H8

Structural formula:

For n > 3, it makes a difference which carbon the next methyl group is added

For straight-chain alkanes, next methyl always added to an end carbon – structural formula (untwisted) has all carbons in a line

C

H

H

HC

H

H

H

C

H

H

Simple Hydrocarbons - AlkanesCondensed formula helps to see structureButane C4 shown as straight-chain isomer

CH3CH2CH2CH3

Type of Formula

Molecular Structural Ball-and-Stick Space Fill Condensed

CH3CH3

CH3CH2CH3

Hydrocarbons – Straight-Chain AlkanesType of Formula

Name Molecular Condensed

Chapt 21 Hydrocarbons [Selected]

21.1 Introduction to Hydrocarbons

21.2 Alkanes [Straight-Chain Only]

21.3 Alkenes & Alkynes

21.4 Hydrocarbon Isomers [also with O]

Section 21.3 Alkenes & Alkynes

• Name a straight-chain alkene or alkyne from its molecular formula or by examining its structure (up to oct-).

• Draw the structural formula or write the molecular formula of a straight-chain alkene or alkyne when given its name (up to oct-).

Alkenes are hydrocarbons that contain at least one double-bond; Alkynes are hydrocarbons that contain at least one triple-bond.

Alkenes

• Alkenes- hydrocarbons that have one or more double bonds between Carbons

• Unsaturated hydrocarbons• Naming: prefix + ene

Naming and Drawing AlkenesPrefix Name Structure

Eth- Ethene H-CH=CH-H

Prop- Propene H-CH=CH-CH2-H

But- 1-Butene H-CH=CH-CH2-CH2-H

Pent- 1-Pentene H-CH=CH-CH2-CH2-CH2-H

Hex- 1-Hexene H-CH=CH-CH2-CH2-CH2-CH2-H

Hept- 1-Heptene H-CH=CH-CH2-CH2-CH2-CH2-CH2-H

Oct- 1-Octene H-CH=CH-CH2-CH2-CH2-CH2-CH2-CH2-H

Doubles Bonds Can Move, Changes Name of Molecule

Prefix Name Structure

But- 1-Butene H-CH=CH-CH2-CH2-H

But- 2-Butene H-CH2-CH=CH2-CH2-H

Pent- 1-Pentene H-CH=CH-CH2-CH2-CH2-H

Pent- 2-Pentene H-CH2-CH=CH-CH2-CH2-H

Hex- 1-Hexene H-CH=CH-CH2-CH2-CH2-CH2-H

Hex- 2-Hexene H-CH2-CH=CH2-CH2-CH2-CH2-H

Hex- 3-Hexene H-CH2-CH2-CH=CH-CH2-CH2-H

Alkynes

• Alkynes- are hydrocarbons that have one or more triple bonds between Carbons

• Unsaturated hydrocarbons• Naming: prefix + yne• Example: Ethyne (Acetylene) used

as a fuel in welding.

Naming and Drawing AlkynesPrefix Name Structure

Eth- Ethyne H-C=C-H

Prop- Propyne H-C=C-CH2-H

But- 1-Butyne H-C=C-CH2-CH2-H

Pent- 1-Pentyne H-C=C-CH2-CH2-CH2-H

Hex- 1-Hexyne H-C=C-CH2-CH2-CH2-CH2-H

Hept- 1-Heptyne H-C=C-CH2-CH2-CH2-CH2-CH2-H

Oct- 1-Octyne H-C=C-CH2-CH2-CH2-CH2-CH2-CH2-H

Chapt 21 Hydrocarbons [Selected]

21.1 Introduction to Hydrocarbons

21.2 Alkanes [Straight-Chain Only]

21.3 Alkenes & Alkynes

21.4 Hydrocarbon Isomers [also with O]

Section 21.4 Hydrocarbon Isomers

• Define the terms isomer, structural isomer, and stereoisomer.

• Categorize molecular structures as being structural isomers, stereoisomers or as not being isomers.

• Distinguish between geometric (diastereomers) and optical isomers (enantiomers)

• Differentiate between geometric isomers with cis- and trans prefixes.

• Describe describe the structural characteristics that are associated with optical isomers

Some hydrocarbons [and other compounds] have the same molecular formula but have different molecular structures.

Section 21.4 Hydrocarbon Isomers

• Generate isomers of compounds containing oxygen in addition to carbon and hydrogen

Some hydrocarbons [and other compounds] have the same molecular formula but have different molecular structures.

Section 21.4 Hydrocarbon Isomers

Key Concepts• Isomers are two or more compounds with the same

molecular formula but different molecular structures.

• Structural isomers differ in the order in which atoms are bonded to each other.

• Stereoisomers have all atoms bonded in the same order but arranged differently in space; stereoisomers can either be optical isomers (enantiomers) or not (diastereomers)

• Stereoisomers which are non-superimposable mirror images of each other are called optical isomers

• Some diastereomers are geometric isomers; these are associated with carbon compounds containing double bonds

Section 21.4 Hydrocarbon Isomers

Key Concepts• When oxygen is present in a compound with carbon and

hydrogen, isomers can involve hydroxy (-OH), ether (-O-) and carbonyl (C=O) groupings

Formulas for Compounds - IsomersIsomers – different compounds which have the same chemical formula2 main categories: structural (aka constitutional) isomers and stereoisomers (aka configurational)Structural isomer - atoms bonded in different orderStereoisomer – atoms bonded in same order but differ in spatial orientation

Web resources at:http://www.chemguide.co.uk/basicorg/isomermenu.html#top

http://www.brightstorm.com/science/chemistry/organic-chemistry/isomers-stereoisomers/

Geometric (Cis-trans)

Other diastereomers(>1 chiral centers)

Diastereomers Enantiomers(optical)

Types of Isomers

Structural (Constitutional)

Stereoisomers(Configurational)

All Isomers

Butane – Structural Isomers

Butane, C4H10 - smallest alkane to have isomers (has two)

Unlike construction of ethane and propane, choice of which H in propane to replace with a methyl group makes a difference

Two possible choices generate two structural isomers – carbons connected to each other differently

Structural isomers differ in physical and chemical properties

propane (C3H8)

H C

H

H

C

H

H

C H

H

H

C H

H

Hor

Making Butane (C4H10) From Propane

H C

H

H

C

H

H

C C

H

H

H

H

H

H C

H

H

C

C

H

C H

H

H

H H

H

Two Equivalent n-Butane Molecules

Right hand structure is twisted version of left hand molecule

Butane - Bond Rotations (not isomers)

Butane C4H10.

=

=

Not isomers – carbons connected in same way and forms can convert from one to the other by rotating around a bond

Structural isomers – carbons connected in different way; bond must be broken to convert one form into the other

Structural Isomers of C4H10

Butane, BP = 0°C Isobutane, BP = -12°C

C C C C

H

H

H H

H H

H H

H

H

C

H

H

H

H H

H

C HC

CH H

H

C

H

C

H

H

H

C

H

H

C

H

H

H

H

C

H

C

H

H

H

C

C

H

H

H

H

H

H

C

H

C

H

H

H

C

H

H

C

H

H

H

H

C

H

C

H

H

H

C

C

H

H

H

H

H

H

Butane (C4H10)

H C

H

H

C

H

H

C C

H

H

H

H

H

H C

H

H

C

C

H

C H

H

H

H H

H

OtherSkeletal Formula

Structural Formula

CH3CH2CH2CH3

CH3CH(CH3)CH3

n-butane straight chain

iso-butane branched

n-Butane CH3CH2CH2CH3

C C

H

H

H

H

H

C

H

H

C

H

H

HH

H

H H

H HH H

HH

H C

H

H

C

H

H

C C

H

H

H

H

H

H C

H

H

C

C

H

C H

H

H

H H

H

Making Pentane (C5H12) From Butane

At first glance, appear to be 4 isomers - but 3 isomers remain

The 3 Structural Isomers of Pentane

C C C C CH

H H H H H

H

HHHHH

C CH

H H H

HHHH

C HC

H H

H

C

C CH

H H

HH

H

H H

H

C

C

H H

H

C

n

Structural Isomers of C5H12 (Pentane)

n-pentane isopentane neopentane

Longest continuous carbon chain:pentane 5 isopentane 4 neopentane 3

Alkane Isomers – Alternate Strategy

Rather than building new isomers by adding a methyl group to known isomers of a particular alkane, the following slides illustrate an alternative approach

1) Draw straight chain version of alkane of interest

2) Break one or more carbon-carbon bonds and rearrange the pieces

3) Check that new molecules don’t repeat existing ones (reflection, rotation)

Structural Isomers of Hexane (C6H14)

C C C C C C

C C C C C

C

C C C C C

C C C C C

C

C

C C C C

C C

Start: connect carbons in a line

Break bonds & rearrange to get other isomers

C C C C

C

CH

H

H

H H H

H

H

H

HHHH

H

C C C C

C

C

H

H

H H H

HHHH

H

H

H

H

H

C C C C C CH

H

H

H

H

H

H

H

H

H H

H

HH

C C C C

C

H

H

H H

HH

HH

H

HH

C H

H

H

C C C C C C

C C C C C

C

C C C C C

C

C C C C

C

CC C C C

C C

C C C C

C

H

H

H H

H

CHHH

HH

H

HH

H

Hexane (C6H14) Isomers Showing H Atoms

C C C C

C

CH

H

H

H H H

H

H

H

HHHH

H

C C C C

C

C

H

H

H H H

HHHH

H

H

H

H

H

C C C C C CH

H

H

H

H

H

H

H

H

H H

H

HH

C C C C

C

H

H

H H

HH

HH

H

HH

C H

H

H

C C C C

C

H

H

H H

H

CHHH

HH

H

HH

H

Hexane (C6H14) Isomers In Skeleton Form

Structural Isomers of Hexane (C6H14)

1

1

1

2

2

2

3

4

3

3

4

4

5

5

5

Boiling Points - Hexane Isomers

Structural Isomers of Hexane (C6H14)Site has rotatable models with display options

http://www.creative-chemistry.org.uk/molecules/hexane.htm

Molecular Formula

Possible # Isomers

Molecular Formula

Possible # Isomers

C4H10 2 C11H24 159

C5H12 3 C12H32 355

C6H14 5 C15H32 4,347

C7H16 9 C20H42 366,319

C8H18 18 C30H62 4,111,846,763

C9H20 35 C40H82 62,481,801,147,341

C10H22 75

# of Alkane (CnH2n+2) Structural Isomers

Geometric (Cis-trans)

Other diastereomers(>1 chiral centers)

Diastereomers Enantiomers(optical)

Types of Isomers

Structural (Constitutional)

Stereoisomers(Configurational)

All Isomers

Enantiomers - Optical IsomersTied to concept of non superimposable mirror imagesFamiliar example – hands: left & right hands are mirror images but do not superimpose

63

Some molecules are like socks - two socks from pair are mirror images that are superimposable (sock and its mirror image are identical)

Molecule/object that is superimposable on its mirror image is achiral; it is chiral if it is not superimposable on its mirror image

Superimposable Mirror Images

Superimposable Mirror ImagesAll molecules have a mirror image – but for many molecules it is the same molecule (achiral)

fluoromethane

H

CH F

H

H

CHF

H

65

Achiral Examples

Nonsuperimposable Mirror Images

Mirror image cannot be rotated so all its atoms align with same atoms of original molecule – i.e., mirror image is not superimposable on original

Non-Superimposable Mirror ImagesOnly when C attached to 4 different groups

When this occurs, C variously referred to as a chiral center, asymmetric carbon, stereogenic center, or stereocenter

Examine each tetrahedral carbon atom and look at four groups (not the four atoms) bonded to itIf groups all different, have a chiral (stereogenic) center

Identifying Chiral Centers

Chiral Molecules - EntantiomersA pair of nonsuperimposable mirror images are called a pair of enantiomers – these molecules will be optical isomers of each other

Chirality – Optical IsomersLouis Pasteur discovered 2 forms of crystallized tartaric acid; forms were mirror images of each other called right and left-handed forms

Non-Superimposable Mirror ImagesChiral molecule – mirror images are enantiomers (optical isomers)

Non-Superimposable Mirror ImagesChiral molecule – mirror images are enantiomers (optical isomers)

OH

CH CH3

COOH

OH

CHH3C

HOOC

(-) lactic acid (+) lactic acidin sour milk in muscles

2-chlorobutane

CH3 CH CH2 CH3

Cl

CH2CH3

CH Cl

CH3

CH2CH3

CHCl

H3C

Non-Superimposable Mirror Images

butan-2-ol

CH3 CH2 CH

OH

CH3

CH2CH3

CCH3 OH

H

CH2CH3

CCH3

HHO

Non-Superimposable Mirror Images

Earlier in presentation, structural isomers of heptane (C7H14) were determined

Some of these isomers are optically active (following slide)

Return to Heptane – Optical Isomers

Structural Isomers of C7H16 (Heptane)

Isomers marked with * have asymmetric carbons (have enantiomers – optical isomers)

*

*

Tro, Chemistry: A Molecular Approach 77

Optical Isomers of 3-methylhexane

Optical isomers have the same physical* and chemical properties except in chemical reactions where chirality is important

Chiral molecules often react differently with other chiral molecules; similar to idea that right hand does not fit a left handed glove – molecule must be correct shape to fit molecule it is reacting with

* Except for their rotation of polarized light

Properties of Optical Isomers

Many natural molecules are chiral and most natural reactions are affected by optical isomerism; e.g. D- & L-amino acids!

Many drugs are optically active, with only one enantiomer being beneficial (or harmful, e.g. thalidomide

Impact of Chirality

Geometric (Cis-trans)

Other diastereomers(>1 chiral centers)

Diastereomers Enantiomers(optical)

Types of Isomers

Structural (Constitutional)

Stereoisomers(Configurational)

All Isomers

Geometric (Cis-trans)

Other diastereomers(>1 chiral centers)

Diastereomers Enantiomers(optical)

Types of Isomers

Structural (Constitutional)

Stereoisomers(Configurational)

All Isomers

Stereoisomers – No Chiral CentersSingle covalent bonds can easily rotate - what appears to be a different structure is not

All structures above are the same (not isomers) because C-C bonds have free rotation

Stereoisomers – No Chiral CentersA double bond between carbon atoms prevents free rotation – structure is locked into place

Stereoisomers – No Chiral CentersGroups on same or opposite sides of double bond – diastereomers (spatially different but not mirror images)

Geometrical IsomersWhen double bonds involved, diastereomers are referred to as geometrical isomers

Although cis and trans modifiers to names are traditional, official naming system uses E and Z modifiers

Isomers with CHO CompoundsWith carbon, hydrogen and oxygen (CHO) present, isomers become more varied

Rule: O in these compounds forms 2 bonds

O found in form of alcohol (COH), ether (COC), or carbonyl (C=O) group

Isomers of C3H8O2 structural isomers in form of alcohol1 structural isomer in form of ether

propan-1-ol

propan-2-ol

ethyl methyl ether

Isomers of C4H10O4 structural isomers in form of alcohol

butan-1-ol

butan-2-ol

2-methylpropan-1-ol

2-methylpropan-2-ol

* chiral center

*

Isomers of C4H10O3 structural isomers in form of ether

1-methoxypropane

2-methoxypropane

diethyl ether

Isomers of C4H10O1 chiral center 2 optical isomers

(2S)-butan-2-ol

(2R)-butan-2-ol

butan-2-ol

*

Isomers of C4H10O

8 total isomers

Summary – Isomers of C4H10O

4 structural isomers in form of alcohol

1 alcohol has a chiral center 2 enantiomers (optical isomers)

5 total isomers in form of alcohol

3 structural isomers in form of ether

0 chiral centers

3 total isomers in form of ether

8 total isomers (7 structural)

Molecular Formula

Possible # Isomers

Molecular Formula

Possible # Isomers

C2H6O 2 C9H20O 405

C3H8O 3 C10H22O 989

C4H10O 7 C11H24O ~2430

C5H12O 14 C12H32O ~6070

C6H14O 32

C7H16O 72

C8H18O 171

# of CnH2n+2O Structural Isomershttp://www.docbrown.info/page07/isomerism1.htm

Structural Isomers

Summary – Isomers