Chapter 3 alkanes

Chapter 3Alkanes, Alkenes & Alkynes

Functional group

• Structural features that make it possible to classify compounds into families

• Group of atoms with characteristic chemical behavior in every molecule it occurs

• Chemistry of every organic molecule regardless of size and complexity is determined by the functional groups it contains.

Alkane

• Saturated hydrocarbons• sp3 hybridized• General formula CnH2n+2

• Referred to as aliphatic (“fat”) compound• Carbons connected in a row are called

straight-chain alkanes or normal alkanes (n-alkanes)

• Branched chain alkanes

• Isomers: Iso+meros– made of many parts– Compounds that have same number and kind of

atoms but different in way atoms are arranged– Constitutional isomers:

• atoms connected differently– Different skeleton– Different functional group– Different location of functional group

• Different compounds different properties but same molecular formula

– Alkane isomers- how C and H might combine• C4H10 2 combinations

• C7H16 9 combinations

• Alkyl groups– Remove 1 H from alkane– Not stable compound themselves– Part of larger compound– Named by replaceing –ane with –yl– Removal of H:• From end of C straight chain alkane straight chain

alkyl• Combine with other functional group• From branched alkyl group

• 1o,2o,3o,4o Carbons• 1o,2o,3o,4o Hydrogens

• Naming alkanes (based on IUPAC – international Union of pure and Applied Chemistry)

Step 1: Find parent HC• Find longest continuous chain of C atoms in the molecule• Use the name of that chain as parent name• Longest chain may not always be apparent• 2 different chains equal length present choose the one

with the largest # of branched point as parent

Step 2: # atoms in main chain• begin at end nearer to 1st branch point• Branch equal distance from both ends begin at end

nearer 2nd branch point

Step 3: Identify and # substituents• Assign # to each substituent to locate point of

attachment to parent chain• If two substituent's on same C, give both same #

Step 4 : Write name as single word• Hyphens to separate different prefixes• Commas to separate numbers• 2@more substituents present cite in alphabetical order• 2@more identical substituents present use multiplier

prefixes (di-,tri-,tetra-… but not used for alphabeticalizing)

• Iso- considered but sec- and tert- not considered

Step 5 : Name complex substituent as though it were itself compound

• Properties of alkanes– Paraffins- little affinity– Alkane little affinity for other substances

• Chemically inert to most lab reagents• Inert biologically and not involved in chemistry of living

organism

– React wit O2, halogens and a few other substances under appropriate conditions

– Reaction with O2 occurs during combustion. CO2 & H2O formed.

– Reaction with Cl2 occurs when mixture is irradiated with UV light. Mixture of chlorinated products occur depending on amount of reactant and time

– Boiling point and melting point increases with molecular weight due to presence of weak dispersion forces between molecues

– Sufficient energy applied to overcome forces melt @ boil

– Higher branching lowers alkanes boiling point due to lower/ smaller surface area lower dispersion forces.

• Conformation of ethane• Stereochemistry: 3D aspect of molecule– Bond rotation around C-C bond can occur freely

due to cylindrical symmetry of s bond– changing spatial relationship between H’s and C’s– Different arrangements of atoms from rotation are

called conformation– Molecules with different conformation

conformational isomers, conformers (cannot be isolated interconvert too rapidly)

– Conformers represented• Sawhorse representation: view C-C bond from an

oblique angle and indicate spacial orientation by showing C-H bonds• Newman projection: view C-C bond directly end-on

represent 2 C by a circle

– Some conformers more stable than others• Staggered

– Most stable, Lower E, 99%

• Eclipsed – Higher E, less stable, 1%

– 12 kJ/mol barrier to rotation (torsional strain due to orbital overlap)

– Propane 14 kJ/mol barrier • H-H: 2x 4kJ/mol • H-CH3: 6 kJ/mol

– Butane: • Not all staggered/ eclipsed conformation have same E• Low energy anti conformation• Gauche conformation, 3.8 kJ/mol higher in E due to

steric strain• Steric strain :

– Trying to occupy 2 atoms in same space– Repulsive interaction when aatoms are forced closer together

than their atomic radii alow

• CH3-CH3 at 0o: 11 kJ/mol steric + torsional strain