Alkanes

Hydrocarbons are organic structures that contain only carbon and hydrogen

Alkanes are the simplest hydrocarbons with a molecular formula of CnH2n+2

The compound has a root name indicating the number of carbons and the –ane suffix

CH4 methane

What is the structure of methane?

Structure of organic compounds in general indicate how properties are derived

If we know the functional groups present and the structure of the compound, we can often predict the properties of a compound without needing to memorize every possible compound

Structure of Methane

As learned, molecules are made by combining atomic orbitals to form molecular orbitals

Using the outer shell orbitals of methane, the compound results from combining the 2s, 2px, 2py and 2pz orbitals of carbon with the four 1s orbitals of hydrogen

The electronic configuration for atomic carbon thus has a lower energy 2s orbital and three degenerate 2p orbitals that are each orthogonal to the others

The valence electrons will thus have 2 in the lower energy 2s orbital and 1 in each of the degenerate 2px and 2py orbitals

To form the methane molecule, therefore the 1s orbital of each hydrogen must form bonds with the orbitals that have electrons

This bonding model has many problems: 1) implies different energy and bond length of C-H bonds

2) Two of the C-H bonds must have a 90˚ bond angle 3) Have too many electrons in an orbital

Hybridization Model for Bonding

Instead of using atomic orbitals for bonding, a different model considers first hybridizing the atomic orbitals to form “hybridized” orbitals

Same rules apply for combining atomic orbitals to form hybrid orbitals

1) Get same number of hybridized orbitals as starting atomic orbitals used to form hybrid

2) Shape of hybridized orbitals is obtained by the mathematical addition of the wave functions for the atomic orbitals

The name (designation) of hybridized orbitals merely refers to the number and type of atomic orbitals used in the formation

sp Orbital

Combine one s orbital with one p orbital

If the orbitals are subtracted then an identical hybridized orbital is obtained directed 180˚ from the first

Bonds formed from the two sp hybridized orbitals will thus have a 180˚ bond angle

Notice the relative shape difference between bonding

and antibonding lobes -Allows more overlap!

sp2 Hybridization

- Can also hybridize by combining one s orbital with two p orbitals (would allow formation of three covalent bonds – one from each sp2 hybridized)

All three sp2 orbitals are in the same plane (large lobe used for bonding 120˚ apart from one another)

Look in the x-y plane, pz is coming in and out of the plane

three sp2 hybridized orbitals

sp3 Hybridization

To form four equivalent bonds carbon can hybridize all of its valence orbitals (three p and one s to form four sp3 hybrids)

The four sp3 hybridized orbitals have a bond angle of 109.5˚

Forms a tetrahedral geometry

All bonds can thus have the same bond length and angle, unlike the model using atomic orbitals

Hybridization Model for Bonding

When a hybridized orbital is used to form a bond with an atom, a new bonding and antibonding molecular orbital are formed

These bonds have the electron density cylindrically symmetric about the internuclear axis

Bonds that are symmetric about the internuclear axis are called sigma (σ) bonds

Sigma bonds and lone pair of electrons (if they are not involved in resonance) use hybridized orbitals for the electrons

When 2nd row atoms have the same substituents, they use sp hybridization for two bonding orbitals, sp2 hybridization for 3 bonding orbitals, and sp3 hybridization for 4 bonding orbitals

Knowing the structure thus allows chemists to predict the hybridization and also the geometry for the compound

Bonding in Unsymmetrical Compounds

In methane there are 4 identical bonds between carbon and each of the four hydrogens

The carbon atom thus adopts a sp3 hybridization and each H-C-H bond angle is 109.5˚ for a perfect tetrahedron geometry

When one of the C-H bonds is replaced with a different atom, however, the perfect tetrahedron geometry is no longer present

(The C-Br bond length is obviously longer than the C-H bonds, thus not a tetrahedron)

We still approximate the carbon as being sp3 hybridized, it is very close as seen by geometry, but we realize this is an approximation

Variable Hybridization

As seen, the hybridization affects the geometry of a compound

Atomic orbitals need not be “hybridized” in integer numbers, need not add exactly one s orbital with 2 p orbitals to yield exactly a sp2 hybridized orbital

As the amount of s and p orbital ratios are changed, the geometry changes

Pure s sp sp2 sp3 Pure p %s 100 50 33 25 0 %p 0 50 67 75 100

Bond < ~ 180˚ 120˚ 109.5˚ 90˚

As %p increases in a hybridized bond, the bond angle decreases

As %s increases in a hybridized bond, the electrons are held closer to the nucleus (since s orbitals are closer to the nucleus on time average than p orbitals)

The geometry is thus intimately related to the hybridization of the atom

Drawing Organic Compounds

Organic chemists use a wedge and dash line system to designate stereochemistry

Wedge line – object is pointing out of the plane Dash line – object is pointing into the plane

H

H HH

To draw a tetrahedral carbon: 1) Make a V with an angle approximately at 109.5˚ 2) Place the wedge and dashed lines in the obtuse angle space

Common errors: 1) placing dashed and wedge lines in acute space 2) Placing either two bonds as wedge or dashed with two bonds in plane 3) Placing dashed and wedge bonds on opposite sides of bonds in plane

Reactive Intermediates

Methyl groups (CH3) if not attached to a fourth atom can form reactive intermediates (not stable structures, but rather intermediates along a reaction trajectory)

Methyl anions Methyl anions are formed if a hydrogen is abstracted from methane by a base

H

H

HH

B

H HH

carbon anions are approximately sp3 hybridized, Lone pairs go into hybridized orbitals

(if not involved in resonance)

Methyl cations

Methyl cations have only 6 electrons in the outer shell of carbon

H

H

HH H

HH Carbocations are sp2 hybridized,

the H-C-H bond angle is 120˚

Methyl radicals Methyl radicals have 7 electrons in the outer shell of carbon

HHH Radicals are also assumed sp2 hybridized,

the H-C-H bond angle is 120˚

H

H HH

Conformational Analysis of Alkanes

- Physical properties of molecules are determined by intermolecular forces (forces between molecules)

- The internal structure of a given molecule can affect the energy due to sterics (intramolecular interactions)

Conformer: different arrangements in space resulting from the rotation of bonds (bonds are not broken when interconverting between conformers)

Consider Methane

H

H HH

No conformers possible; methane has a given energy value that does not change (any rotation about the equivalent C-H σ bonds yields the same structure

in three-dimensions) *this is not the case with any higher hydrocarbon homologue

Conformational Analysis of Ethane

Structures have different energy due to different arrangements in space (hydrogens have different spatial arrangements in different conformers)

H

HHH

HH

H H

HH

HH

H

HHH

HHRotate 60˚ Rotate 60˚

Newman Projections

- Convenient way to view conformational analysis

In Newman Projections change view by looking down one carbon-carbon bond

In order to distinguish the front atom from the back atom in a Newman

projection, the substituents are attached to a point for the front carbon and to a

circle for the back carbon

H

HHH

HH

C

H

H

H

H

H

H H

H

H

H

H

HView down C-C bond

Which hydrogens are attached to front

or back carbon?

To Draw Newman Projections

1) Determine which bond is being considered

2) Determine which atom is front atom of bond being considered

3)  The substituents attached to the front atom are drawn to a point, the substituents attached to the back atom are drawn to a circle

4) The relative angles and orientation of the substituents are maintained

H3CCH3

CH3

H HCH3

HH

1) Look down the C2-C3 bond of butane

2) Assign front and back atom of bond

3) 2 hydrogens and one CH3 group are attached to both front and back carbons

4) Draw Newman projection maintaining bond orientation

Consider butane

Front atom of C2-C3 bond

Newman projections of ethane conformations

Newman projections demonstrate energetic and spatial interactions of conformers

H

H

HHH

HH

H

H

H

H

H

H

H

H

H

H

H

Rotate 60˚ Rotate 60˚

staggered eclipsed

Substituents are as “staggered” as

possible

Substituents are “eclipsing” each other

Substituents have returned to a “staggered” conformation

Eclipsed conformations are higher in energy

One cause is the sterics As the substituents that are eclipsed become larger, the energy of the conformer raises

Consider the space filling volume of atoms

CH3H3CH3C

CH3

staggered eclipsed

Conformational Energy Diagram for Propane

The energy difference between staggered and eclipsed conformations is larger for propane versus ethane due to the greater steric interaction between larger methyl and hydrogen

Ener

gy

torsional angle0˚ 60˚ 120˚ 180˚ 240˚-60˚

Different Types of Interactions Arise with Larger Carbon Frameworks

Consider butane looking down the C2-C3 bond

CH3

H HCH3

HH

anti

H

H

CH3

CH3

HH

H

H CH3CH3

HH

H

H3C

H

CH3

HH

H3C

H

H

CH3

HH

gauche

totally eclipsed

eclipsed

“gauche” conformation is higher in energy than anti

(both are “staggered” conformations)

“totally eclipsed” conformation (which has

largest groups eclipsing each other) is higher in energy than other eclipsed conformations

Naming Conventions for Alkanes

Straight Chain Alkanes

The alkanes are named according to the number of carbon atoms in the chain

Ends with an –ane suffix

Root name # of carbons (n) H-(CH2)n-H

Meth- 1 Eth- 2 Prop- 3 But- 4 Pent- 5 Hex- 6 Hept- 7 Oct- 8 Non- 9 Dec- 10

All alkanes have the empirical formula CnH(2n+2)

Origin of Naming for Alkanes

C1 through C4 are result of common names for carbon chains, C5 through C10 are named due to the Greek word for their root

(an 8 sided circle for example is an octagon – OCT represents 8)

Meth - means wine or spirit in Greek, yl – means wood or matter in Greek

Therefore methyl alcohol (which has one carbon) means a spirit from wood Methanol is obtained from distillation of wood (sometimes called wood alcohol)

METH is thus kept for a 1 carbon chain, yl is kept to mean a carbon group and is used for any carbon substituent

(methyl, ethyl, propyl, etc.)

ETH root comes from Greek word ether (to shine) Shine → sky → colorless liquid

Ether (also called diethyl ether) is a colorless liquid and it has two 2-carbon chains a two carbon chain is ETH

PROP common name is a result of the three carbon chain acid called propionic acid

Protos (Greek for first), pion (Greek for fat) Propionic acid thus literally means “first fat”

1 carbon acid is formic acid (from ants) 2 carbon acid is acetic acid (from vinegar)

Both formic acid and acetic acid are soluble in water due to the low carbon content, Propionic acid is thus the smallest acid chain that is not soluble in water but soluble in

organic solvents (thus first fat – fatty acids are long chain carboxylic acids)

BUT comes from the common name for a 4 carbon carboxylic acid (butyric acid) Butyric acid is the cause for the smell in rancid butter

(where BUT comes from the word for butter)

Origin of Naming for Alkanes

IUPAC Nomenclature Procedure for naming carbon chains containing branches or substituents (non-straight chain)

1)  Find the longest continuous carbon chain in the structure -this determines the root name

2) Any carbon not on this continuous chain is a substituent (appendage)

3) Number the main chain starting from the end closest to the first substituent

4)  The substituents are still named according to the number of carbons (the suffix for a substituent is –yl instead of –ane)

-CH3 methyl -CH2CH3 ethyl

5) Place all substituent names before the root name in alphabetical order

6) The substituent must be numbered to indicate the point of attachment to the main chain

7) Group multiple substituents of the same kind together and label di-, tri-, etc.

8)  When alphabetizing, the prefixes di-, tri, n-, t- are ignored (the only prefixes used for alphabetizing are iso- and neo-)

9)  With a ring compound the number of carbons in the ring determines the root name with a cyclo- prefix

Common Names for Alkyl Substituents

Many alkyl substituents have common names

Consider propyl

There are two ways an alkyl appendage with three carbons can be attached

Any straight chain appendage has the n- prefix (for normal)

CH3CH2CH2- n-propyl

This distinguishes the straight chain compound from the other isomer

Isopropyl (1-methylethyl) using IUPAC

Use iso prefix (short for isomer)

CH

H3C

H3C

With larger alkyl substituents, more possibilities for isomers exist

Consider butyl

H3CH2CH2CH2C

CH

H3C

H3CH2C

CH2CH

H3C

H3C

C

CH3

H3C

CH3

n-butyl

isobutyl

secbutyl (s-butyl)

tertbutyl (t-butyl)

Common Names for Alkyl Substituents

The sec- and tert- prefixes for common names are based upon degree of substitution A carbon bonded to three other carbons is called a tertiary carbon

A carbon bonded to two other carbons is called a secondary carbon

A carbon bonded to one other carbon is a primary carbon

To name substituents, only consider the bonding pattern of the carbon directly attached to the main chain, and then consider how many other carbons are bonded to that carbon to obtain

tert- or sec- names

e.g. tertbutyl

CH

H3C

H3CH2C

secondary carbon(2˚)

C

CH3

H3C

CH3tertiary carbon(3˚)

CH2CH

H3C

H3Cprimary carbon

(1˚)

e.g. secbutyl

e.g. both n-butyl and isobutyl

Complex Alkyl Groups

As the alkyl substituents become more complicated (e.g. more branching) the same IUPAC rules are followed and the name for the whole appendage is placed in parenthesis

The root is the cyclooctane ring (usually the ring is used as a root although if the number of carbons in the substituent become

larger then the ring could be named as a substituent)

ethyl substituent 1,1,3-trimethylbutyl substituent

(with substituents need to count from the carbon at the attachment to root and find longest chain)

After alphabetizing: 1-ethyl-3-(1,1,3-trimethylbutyl)cyclooctane

Cycloalkanes

Alkanes that are bonded in a ring structure are called cycloalkanes and have a molecular formula of CnH2n

Naming of cycloalkanes follows the same rules for acyclic alkanes, just need to place the word “cyclo” in front of the root name

One difference with polysubstituted cycloalkanes, however, is that the substituents have a stereochemical nonequivalence depending upon placement

CH3CH3 CH3

CH3

1,2-dimethylcyclopropane

cis Substituents on same side of ring

trans Substituents on opposite sides of ring

Attractive Forces in Alkanes

- Type of electron interaction between molecules determine the physical properties

Na

Na

Na

Na

NaH3C

O

O

H3CO

OH3C

O

O

H3CO

OH3C

O

O H3C Cl!+ !-

H3C Cl!+ !-

CH3Cl!+!-

CH3Cl!+!-

Coulombic attraction (energy caused by interaction of point

charges)

Dipole-Dipole interactions

(alignment of partial charges due to bond

dipoles)

van der Waals forces (energy caused by induced dipoles at nonpolar surfaces)

!!-

!!+!!-

!!-!!+

!!+

Nuclear Magnetic Resonance of Alkanes

The most common tool for an organic chemist to determine structure

Any nucleus with either an odd atomic number or odd mass has a “nuclear spin”

A charge species that is spinning creates a current loop, which in turn creates magnetic field lines

+ + + +

proton

Spinning direction

Spinning nucleus creates a current loop

Current loop creates magnetic field lines

I B

This magnetic field is analogous to a bar magnet

Thus a spinning nucleus behaves similar to tiny bar magnets

Nuclear Magnetic Resonance

B

+

B

N S

In the presence of an external magnetic field the spinning nucleus can align in one of two ways (with or against the field) when S = ½

For any S = ½ nucleus, obtain two quantized spin directions in large external magnetic field (B0)

Most nuclei of interest for organic chemists have a S = ½ state (1H, 13C, 19F)

B0

Nuclear Magnetic Resonance

N S

α-spin state, align with field

β-spin state, align against field

Resonance

A key remaining point is the nucleus in the α-state can be changed to the β-state

-need the exact amount of energy that separates the two states

We know this energy difference (ΔE)

We can supply this energy by absorption of the appropriate wavelength of light ΔE = hν

In NMR we supply energy in the radio frequency region to cause the transition of nuclei from the α-state to the β-state

This absorption of energy will cause the spin state to change

In presence of external magnetic field the spin states are quantized with more spins in the α-state than the β-state

With exact combination of external magnetic field (B0) and energy of photons (ΔE, which happens to be in the radio frequency for NMR) the nuclear spins can flip from the

α-spin to β-spin state, thus they are in “resonance”

Called nuclear magnetic resonance (NMR)

Nuclear Magnetic Resonance

N S

N S

N S

N S

B0

α-spin state

β-spin state

N S

N S

N S

B0

α-spin state

β-spin state ΔE = hν

Shielding

Need to remember the structure of a compound (consider only an isolated C-H bond)

To reach the nucleus the magnetic field must past through the electron cloud surrounding the nucleus

The electrons surrounding the nucleus are charged species that can rotate in the presence of the external magnetic field

What this means is that the external magnetic field (B0) is effectively reduced by the time it reaches the nucleus (B0 minus the field of the electron cloud)

C HB0

Bnet = B0 - Belectron

The NMR spectrum thus will be able to distinguish the number of either hydrogens or carbons in a different electronic environment (no matter how small the difference)

Signals in a NMR Spectrum

H3C CH3

H H

Propane has two different kinds of carbon atoms (the two primary carbons and the one secondary carbon)

Each will give a different signal in a 13C NMR

Likewise in a 1H NMR there will in addition be two signals due to the primary and secondary hydrogens

Unlike a 13C NMR, however, it is relative easy to integrate the signals in a 1H NMR to determine not only that there are two different types of hydrogens,

but also that there is 3X as many hydrogens of one type than the other