Properties of Alkanes
Physical
Homologous series-members all have same general formula (for Alkanes, CnH2n+2, where n is the number of carbon atoms), always increases by CH2
Alkanes are intermolecularly bonded by London forces and are nonpolar
Due to being nonpolar, they are insoluble in water.
Boiling points increase as mass increases (London forces become stronger with more mass)
Alkane_boiling_points.gif


Form structural isomers (compounds with same number of atoms of each constituent element, but atoms are arranged differently)
Isomer boiling points decrease with a more spherical structure or more branching side chains
Reason: Less surface area of interaction between molecules
Analogy: Handshake vs Hug-Which is harder to break? The hug would be harder to break given the same force because there is a greater area of contact.

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Chemical
Reacts with oxygen to form carbon monoxde or carbon dioxide and water
Reacts with halogens
Relatively inert (unreactive), most unreactive group of Organic Compounds.








Reactions and Reactivity of Alkanes

Alkanes are the least reactive of the organic compounds, and are relatively inert in nature. However, there are two main reaction types that alkanes undergo:

Combustion
Complete-An alkane reacts with Oxygen, producing carbon dioxide and water. This has been a vital source of energy for the human civilization.

Example:
CH4+2O2-->CO2 + 2H2O

Note: If there is insufficient oxygen, incomplete combustion occurs, carbon monoxide, carbon, and carbon dioxide will be formed.

Halogenation
An alkane reacts with a halogen. This is a substitution reaction, where a Carbon-Hydrogen bond is broken and a Carbon-Halogen bond is formed. There are many possible products that can be formed out of two reactants, with the type of product depending on the amount of halogen.

Example: CH4+Cl2-->CH3CL+HCl
If there is enough halogen, the hydrogens gradually will all gradually get replaced, forming carbon tetrachloride.

The halogens that most frequently react with alkanes are chlorine and bromine.



Free Radical Mechanism
An alkane and either chlorine or bromine in the presence of ultraviolet energy. This is a three-step process

1. The halogen, as it is diatomic, is separated by the UV energy to individual atoms. Halogens having 1 unpaired electron, become free radicals. (Initiation)


2. Hydrogen atoms are gradually removed and replaced by halogen atoms, some Hydrogen atoms bond to halogen atoms. Some free radicals may collide with hydrocarbon groups, forming a compound with Hydrogen, and also another free radical. (Propagation)


3. Reaction stops, radicals combine with each other, or escape, or react with another radical to form a non-radical product. (Termination)

030201_030_radical_halogenation.jpg




Alkenes
-Contain a double bond between two carbon atoms
-Formula of CnH2n
-Nonpolar (bonded together by London Forces, which increase with mass)
-Insoluble in water, soluble in nonpolar solvent
-More branching decreases boiling point
-Significantly more reactive than alkanes

-Unsaturated (not singly bonded to 4 atoms, but has one double bond), can be made saturated by reaction that converts a Carbon-Carbon double bond into two single bonds between carbon and other atoms


Reactions of Alkenes
1. Combustion
-Same as alkanes, where complete combustion forms carbon dioxide and water, while incomplete combustion forms carbon, carbon monoxide, carbon dioxide, and water

2. Hydrogenation
-Reaction must be catalyzed by nickel at a temperature of 180 degrees celsius
-Forms an alkane with the same number of carbon atoms

ex.
C2H4+H2--->C2H6

(To denote catalyst/condition, simply write the catalyst and condition above the arrow. In this case, write Ni, 180 degrees celsius)

3. Halogenation
A. With Halogens
-No need for Free Radical Mechanism
-C-C double bond is broken, previously double bonded carbons are bonded to one halogen atom each
-simply, both halogen atoms in a molecule are combined into the alkene
-Forms an alkyl halide (dihalogenoalkane)
-Di-substituted product (i.e. 2 hydrogens in equivalent alkane are replaced by halogen atoms)

ex.
C2H4+Br2--->C2H4Br2
Ethene+Bromine gas--->1,2-dibromoethane

B. With Hydrogen Halide
-One of the formerly double bonded carbon atoms is bonded to a new hydrogen atom, the other is bonded to a halogen atom
-Forms a halogenoalkane
-monosubstituted product

ex.
C2H4+HCl--->C2H5Cl
Ethene+Hydrogen chloride--->Chloroethane

4. Hydration
-Water, under normal conditions, will not react with an alkene
-With the use of sulfuric acid, hydrogen sulfate is added to the alkene
-The product can then react with water, forming an alcohol and recovering the sulfuric acid

ex.
(H2O)
C2H4+H2SO4--->C2H6O4S--->C2H5OH+H2SO4
Ethene+Sulfuric Acid--->Ethyl hydrogensulfate (reacts with water)--->Ethanol+Sulfuric Acid

5. Polymerization
-Alkenes form bonds with each other
-Happens under certain conditions
-Formed by several bonded units of one type of alkene
-Forms an alkene polymer

ex.
nC2H4--->(C2H4)n where n is the number of ethene units



Carboxylic Acids
-Contain a COOH (carboxyl) functional group
-Naming: a. Count number of carbons in longest chain and use appropriate prefixes.
b. Same rules for side chains apply as in Alkanes and Alkenes
c. In naming, the carbon atom in the COOH group is counted as the 1st, and must be part of the parent chain
d. Name pattern-prefix of parent chain+oic acid
-General Molecular Formula: CnH2nO2
-Molecular formula can either be written following the general molecular formula above, or like this: Cn-1H2n-1COOH

General properties
-Intermolecular force: Hydrogen bonding since there are oxygen and hydrogen atoms present
-Polar
-Soluble in water
-Relatively high boiling points
-pH lower than 7 (obviously as they are acids)


Aldehyde and Ketone
-functional group of C=O (carbonyl)
-polar
-Intermolecular force exhibited: Dipole-dipole (Hydrogen and Oxygen atoms not directly attached, disallowing hydrogen bonding)
-moderate boiling point (higher than equivalent alkane/alkene but lower than equivalent alcohol or carboxylic acid)
-Soluble in water but solubility drops as parent chain becomes longer

-Differences of Aldehyde and Ketone:
In an aldehyde, a hydrogen atom is attached to the carbonyl group, making it easy to oxidize
In a ketone, there is no hydrogen atom attached to carbonyl, and it can only be oxidized by an agent which can break carbon-carbon bonds
-Naming: same general rules governing prefixes, parent chains, side chains; aldehydes end in -al while ketones end in -one.
-Naming (continued): For ketones, use a number that denotes the order at which the C atom in the carbonyl group appears in the parent chain. For aldehydes, the Carbon in the carbonyl group is always considered the 1st in the parent chain.



Halogenoalkane (A.K.A. Alkyl Halide)
-formed by halogenation of alkanes or alkenes, halogens are bonded to carbon atoms
-polar (except if iodine)
-dipole-dipole attraction (except iodine)
-moderately low boiling point (with iodine: low boiling point)
-slightly soluble in water, fully soluble in nonpolar solvent
-Naming: before the parent chain's name, put as a prefix the halogen in the compound, noting the number of halogen atoms within the compound, same rules apply for side chains as in alkanes and alkenes. If the side chain is bonded to halogens, use the same rule as the parent chain
-Naming (continued): if more than one halogen, name alphabetically. Also, denote the least count of the carbon atom which the halogen is bonded to.

Types of Alkyl Halides
a. Primary
Carbon that halogen is bonded to is attached to one alkyl group

haloalkanep.gif

b. Secondary
Carbon that halogen is bonded to is attached to two alkyl groups

haloalkanes.gif



c. Tertiary
Carbon that halogen is bonded to is attached to three alkyl groups

haloalkanet.gif



Substitution Reaction with NaOH
-Alkyl halide reacts with NaOH to produce an equivalent alcohol (same chains) and a salt
-OH replaces halogen atom
-Equation: Alkyl halide + NaOH-->Alcohol + Salt

ex. 2-bromopropane + Sodium Hydroxide-->2-Propanol + Sodium Bromide

external image substprop.gif



Amines
- NHn functional group (n denotes number, which can be from 0-2)
-Alkane reacts with ammonia, hydrogen atoms in ammonia are replaced by alkyl groups
-Intermolecular force: Hydrogen bond (Primary and secondary); dipole-dipole (Tertiary)
-Polar
-Moderately low (Tertiary)-Relatively high (Primary and Secondary) boiling point
-soluble in water
-As they increase in mass, the odor they emit gets more foul
-Types of Amines:
a. Primary

external image primstructs.gif


NH2 is bonded to one alkyl group
Naming: Same rules on side chains as alkanes and alkenes; Parent chain rule is basically the same, except you add the least count of the carbon atom which the NH2 is attached to followed by the parent chain's name, preceded by the prefix "amino". You may also mention the parent chain's name first then add the suffix "-amine".

external image primnames2.gif




b. Secondary
- NH group is attached to two of the same kind of alkyl group or two different kinds of alkyl groups
-Naming: Use the prefix di- then name the alkyl group the NH is attached to, the main name should end with -amine (same alkyl group)
List the two alkyl groups the NH is attached to in alphabetical order. (different alkyl group)

external image secstructs.gif


c. Tertiary
-N is attached to three of the same alkyl group or different kinds of alkyl groups
-Naming: same as secondary, except di- is replaced by tri- (same alkyl group)
same as secondary, except you have to name three alkyl groups. If there is a doubling of one alkyl group, use di- (different alkyl groups)

external image tertstruct.gif







Alcohols
- OH functional group
-Naming: a. Count number of carbons in longest chain and use appropriate prefix, change the end to -ol
b. Same side chain naming rules as in Alkanes and Alkenes
c. Identify the least count of the carbon atom the -OH group is attached to, putting the number before the parent chain (connected via hypen) or separating the parent chain's name into prefix + number + -ol
-General molecular formula: CnH2n+1OH
-molecular formula is written following above format


Kinds of Alcohols
a.Primary
-Carbon atom that is to -OH group is bonded to one alkyl group (CHn), exception of Methanol

alcoholp.gif


b. Secondary
-Carbon atom that is connected to -OH group is bonded to two alkyl groups

alcohols.gif

c. Tertiary
-Carbon atom that is to -OH group is bonded to three alkyl groups

alcoholt.gif





General Properties
-Exhibit hydrogen bonding (O and H present)
-Relatively high boiling points
-soluble in water, but solubility decreases as parent chain becomes longer
-Polar
-neutral substance

Reactions

a. Combustion
Like all other organic compounds, can be complete or incomplete
Complete produces Carbon dioxide and water
Incomplete produces carbon, carbon monoxide , carbon dioxide, water

b. Oxidation
-loss of electrons
-give off atoms

Oxidizing Agent
-A compound that gains electrons in an oxidation reaction

For oxidation, alcohols usually react with dichromate (Cr2O7)

Primary alcohols
-can be oxidized to either an aldehyde or carboxylic acid, depending on the reaction conditions

i. Partial oxidation
-alcohol becomes an equivalent aldehyde (same parent chain, side chains)
-just like incomplete combustion, there is not enough of the inorganic reactant
-Hydrogen dissociates from oxygen in the OH functional group
-happens under distillation

Equation:
Alcohol + Dichromate ion-->Aldehyde + Chromium (III) + Water

ex. Ethanol + Dichromate ion-->Ethanal + Chromium (III) + Water

external image padding.gif

makealdionic.gif



ii. Full Oxidation

-alcohol becomes an equivalent carboxylic acid carboxylic acid
-enough inorganic reactant
-Hydrogen dissociates from oxygen in OH functional group
-happens under reflux (vapors turn back into liquid to react again)

Equation:
Alcohol + Dichromate ion-->Carboxylic acid + Chromium (III) + Water

ex. Ethanol + Dichromate ion-->Ethanoic acid + Chromium (III) + Water

makeacidionic.gif



Secondary alcohols
-can only be oxidized to ketones

Equation:
Alcohol + Dichromate ion-->Ketone + Water

ex. 2-propanol + Dichromate ion-->Propanone + Water

makeketdiag.gif
Tertiary Alcohol-no reaction


Nucleophilic Substitution
Nucleophile-Anions or molecules with lone electron pairs
Happens in halogen alkane reactions with Hydroxide (OH-)
Molecularity-Number of molecules that are involved in a reaction, determining its rate
Unimolecular-One molecule is involved in the reaction's rate
Bimolecular-Two molecules are involved in reaction rate
Leaving group: atom that separates/leaves (halogen)

SN1 Reaction-Happens for Tertiary Haloalkanes-Stepwise (processes occur in a step-by-step fashion)Step 1 (slow step): Carbon atom gives electron to halogen atom, halogen atom dissociates from carbon, with the halogen being negatively charged and the carbon being positively charged. When the halogen leaves, space is opened up for the nucleophile to attack anywhere. Products in this step: Carbocation and halogen anion (as leaving group) Step 2 (fast step): positively charged carbon bonds with negatively charged OH, forming equivalent alcohol-Resulting product can be normal or inverted

SN2 Reaction-Happens for Primary Haloalkanes-Concerted (Steps occur simultaneously)Step 1: OH bonds with carbon atom by attacking it from the back as there is a lack of space in front thanks to the halogen atom. The C-OH bond is forming while the C-halogen bond is breaking Product: ion composed of alkyl group, halogen, and OH-Step 2: The halogen dissociates from carbon as the OH is fully bonded. Product: alcohol and halogen-Product formed is only inverted.

Both Mechanisms have heterolytic fission, wherein the number of electrons is unevenly distributed.


Why Can't Tertiary Alcohols Undergo SN2?
Steric Hindrance: Carbon atom is bonded to three large alkyl groups, giving the OH ion not enough room to attack
Positive Induction Effect: Alkyl groups tend to push bonding pairs of electrons to central carbon atom, spreading the positive charge throughout the molecule. The more alkyl groups the central carbon atom (i.e. carbon atom bonded to either halogen or OH), the more stable the ion in the first step will be. This makes Primary Haloalkanes less likely to undergo SN1.


Trends in Nucleophilic Substitution
Halogens: The lesser the electronegativity, the more likely the halogen will dissociate. The lesser polarity in the bonds makes it less stable.
Nucleophiles: The nucleophiles that are charged, being more likely to donate electrons will more easily bond the greater the charge, and charged nucleophiles will be more reactive than neutral ones. For neutral ones, the more polar the nucleophile is, the more reactive it will be. Nucleophile is better if:a) it has more lone pairs b) it has a negative charge c) it contains pi bonds


To Summarize in a Table

SN2
SN1
Molecularity
Bimolecular
Unimolecular
Steps
concerted, all simultaneous
2: slow step and fast step
Stereochemistry
inverted only
inverted and right-side up
Substrate
primary haloalkane
tertiary haloalkane
Leaving Group
atom (halogen)
anion (halogen)
Intermediate
transition phase with 5 branches
Carbocation


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Image: http://wikipremed.com/03_organic_mechanisms_800/030201_030_radical_halogenation.jpg