Reactions of Hydrocarbons

Reactions of Hydrocarbons

Organic Chemistry Alkanes, alkenes, alcohols, aldehydes and ketones Organic Chemistry Of the 10 million or so compounds that are known today, about 9 million of those are carbon containing compounds Organic Chemistry is the chemistry of these C containing compounds Compounds made of C and H are called hydrocarbons Organic compounds can also contain O, N, S and transition metals Organic Chemistry is the chemistry at work in the cell, so encompasses biochemistry Understanding of the cell requires an understanding of organic chemistry

Hydrocarbons Compounds made up of only the elements carbon and hydrogen R = Alkyl group contains C and H both singly bonded (alkane) 3 Functional Groups Groups of atoms or special bonds within a hydrocarbon that are responsible for the characteristic reactions of the molecule 4

Functional Groups 5 Drawing Structures There are many ways to draw or write a chemical formula/structure for a given organic molecule Consider the hydrocarbon decane CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 Your turn Your Turn

8 Naming: The Three Basic Parts The name for any organic molecule consists of three basic parts: Prefixes-Parent-Suffix Each part of the name has a purpose

Parent: many carbons in the longest chain Suffix: tells you the main functional group Prefixes tells you about other things attached to the main chain (if any) Basic Part - Suffixes Suffixes on the end of the name of an organic molecule tell you what major family the molecule belongs to

The suffix for an alkane is -ane (example ethane) Alkene = ene (example ethene) Alkyne = yne (example propyne) Ether = ether (diethyl ether) Carboxylic Acid = acid (ethanoic acid) Alcohol = ol (ethanol) Ketone = one (dimethylketone aka acetone) Aldehyde = al (methanal)

Basic Part the Parent The parent part of the name tells you how many carbons are in the main chain of the molecule The main chain of the molecule is defined for alkanes as being the longest chain in the molecule Number IUPAC Name

Condensed Structural formula of C atoms 1 methane CH4 2 ethane

CH3CH3 3 propane CH3CH2CH3 4 butane

CH3CH2CH2CH3 5 pentane CH3CH2CH2CH2CH3 6 hexane CH3CH2CH2CH2CH2CH3

7 heptane CH3CH2CH2CH2CH2CH2CH3 8 octane CH3CH2CH2CH2CH2CH2CH2CH3

9 nonane CH3CH2CH2CH2CH2CH2CH2CH2CH3 10 decane CH3CH2CH2CH2CH2CH2CH2CH2CH2CH3 Parent and suffix

The parent is named based on the number of carbons 1 carbon = meth So a one-carbon alkane is called methane CH4 Parent and suffix 2 carbons = eth So a two carbon alkane is called ethane. CH3CH3

Parent and suffix 3 carbons = prop So a three carbon alkane is called propane. CH3CH2CH3 Basic Part Prefixes Prefixes are the bits and pieces that are attached to the main chain (parent) of the molecule An example of a prefix might be a halide attached to the main chain as in: Br

Prefixes the Halides The family called alkyl halides does not have a suffix. Halides are always named as prefixes. Fluorine is called fluoro Chlorine is called chloro Bromine is called bromo

Iodine is called iodo Putting together a name The rules for IUPAC nomenclature include: Step 1: Find the main chain Step 2: Number the main chain Step 3: Identify all prefixes and their position numbers

Step 4: Write the full name: Prefixes-Parent-Suffix Now lets take a look: Follow the rules Name this molecule Br Find the main chain The longest chain in this molecule has seven carbons and only a halide (which is always named as a prefix). Br

This is heptane. Number the chain This molecule is symmetrical. It doesnt matter if we go left to right or right to left that bromine is attached to carbon #4 Br 4 Final Answer? If you put the prefix first, then parent and suffix, what do you get? Br

4 Bromo hept ane includes all of the pieces, except for indicating where the bromine is attached... 4-bromoheptane is the complete name. What would you do different..? How would you name this one? Br Number this asymmetric molecule by beginning your numbering from whichever end is closest to the first substituent (aka: branch, prefix).

So in this case You would number from left to right, so the bromine winds up on position #3. Br If you numbered from the other end, the bromine would be on #5. Always aim for the lowest possible number on the first substituent! Final Answer? 3-bromoheptane

Br Alkyl Groups Alkyl groups are named similarly to alkanes, based on the number of carbons in the fragment A fragment of methane, CH4, would be CH3 This fragment is called methyl where meth stands for one carbon and yl stands for fragment (alkyl group). Methyl Group The molecule shown here has a 7-carbon main chain.

Notice that it has a branch attached to carbon #4. This branch has a single carbon a methyl group. 4-methylheptane is the name of this compound. Ethyl Groups A two-carbon alkane is called ethane, CH3CH3. The corresponding two-carbon fragment is always CH3CH2-, which is called ethyl. 4-ethylheptane is the name for this one. Propyl Groups There are two possible three-carbon alkyl groups to

form from propane, CH3CH2CH3. The straight chain version: CH3CH2CH2- which is called propyl or n-propyl (where n stands for normal or straight-chained) 2-propyl-butane Your turn Your Turn 31

Nomenclature for Alkenes Alkenes have one or more C=C Step 1: Name the longest chain with the C=C in it Step 2: Number the longest chain so that the carbons in the C=C have the lowest numbers Step 3: Locate the lowest numbered C in the C=C Step 4: Locate and name attached groups Step 5: Combine the names ending with the suffix ene Naming Alkenes 2 carbons prefix eth 1 C=C suffix ene Name = ethene 3 carbons prefix prop 1 C=C suffix ene

Name = propene 4 carbons prefix but1 C=C suffix ene Name = butene But where is the C=C? 1 butene (or but-1-ene) 2 butene (or but-2-ene)

Naming Cycloalkenes 6 carbons prefix hex1 C=C suffix ene Name = hexene 6 carbons in a ring cyclohexene

5 carbons prefix hex1 C=C suffix ene Name = pentene 6 carbons in a ring cyclopentene Your turn

Your Turn 37 Aromatics Aromatics contain This is called benzene This is called toluene (methyl benzene) What is this one called? Ethyl-benzene

Alkane Reactions: Oxidation Alkanes are unreactive because they are so stable All alkanes combust (burn in air) to make carbon dioxide and water CH 4 + 2O2 CO2 + 2H 2O + 890 kJ / mol Very exothermic because O-H and C=O bonds are much more stable than C-C and

C-H bonds If there is not enough oxygen one gets incomplete oxidation of the alkane 2CH 4 + 3O2 2CO + 4H 2O or CH 4 + O2 C + 2H 2O Reaction with O2 (combustion) Combustion can be used to indicate the nature of the hydrocarbon

Heavier hydrocarbons (more C) dont evaporate as easily so less easy to combust Alkynes, Aromatics and cycloalkanes and cycloalkenes burn with sooty flames Ethyne (acetylene) aromatic

methane cyclohexene Your turn Your Turn Do Questions 1&2 41 Alkane Reactions: Halogenation Replacement of one or more H with a Halogen

F, Cl, Br, I Reaction requires the halogen to first break apart into free radicals which can happen when heated or exposed to UV light If there is light present then a series of chain reactions can occur, creating all sorts of products but the reaction is slow Your turn Your Turn Do Question 3 43

Addition Reactions of Alkenes C=C double bonds are generally less stable than C-C X-Y H-F, H-Cl, H-Br, H-I H2, F2, Cl2, Br2,I2 H-OH

Alkene / HCl Thermochemistry The relative instability of C=C = 249 single H-Cl + = 263 double

+H-ClRCH=CH + H2 2C=CH2 three singles H-RCH-CHH-CH Cl-RCH-CH 2-Cl or 2-H? 2-CH 2-Cl The reaction is favorable by 14 kcal/mol

= 57kJ/mol 99 83 103 81 What about selectivity? Is addition of HX across an unsymmetric alkene regioselective or not? 146

Reactions: Alkenes Reaction with Br2,Cl2 etc Br- adds from other side entering a C-Br * antibonding molecular orbital of a bromonium ion (more room) In cyclic compounds you get the trans isomer Reaction is faster than with alkanes Reaction with Br2 (Bromine test) Add alkane or

alkene Alkane Br2 water Alkene Alkenes and alkynes quickly react with Br2 to make haloalkanes (colorless) Alkanes dont react (except slowly in bright light) Benzene doesnt react toluene will Your turn

Your Turn Do Questions 4-7 48 Reactions: Alkenes Reaction with Potassium Permanganate Baeyer test (create diols) Alkane or aromatic (negative test)

In acid Purple In base Ethane-1,2-diol Alkene (positive test) Light Pink

Your turn Your Turn Do Questions 4-7 50 IUPAC Nomenclature Alcohols Find the longest carbon chain containing the carbon with the -OH group

Drop the -e from the alkane name, add ol Number the chain, starting from the end closest to the -OH group Number and name all substituents. => 52

Name these: CH3 CH3 CH CH2OH 2-methyl-1-propanol OH CH3 CH CH2CH3

2-butanol CH3 CH3 OH C OH CH3 2-methyl-2-propanol Br

CH3 3-bromo-3-methylcyclohexanol Naming Phenols When an OH is attached to a benzene ring we call it a phenol -OH group is assumed to be on carbon 1

For common names of disubstituted phenols, use ortho- for 1,2; meta- for 1,3; and parafor 1,4 Methyl phenols are cresols. OH Cl 3-chlorophenol

meta-chlorophenol OH H3C 4-methylphenol para-cresol 54 Alcohols: Classification Alcohols have an OH functional group attached to a C

The reactivity of the alcohol depends on how many C are attached to the C-OH Primary: carbon with OH is bonded to one other carbon Secondary: carbon with OH is bonded to two other carbons

Tertiary: carbon with OH is bonded to three other carbons Aromatic (phenol): -OH is bonded to a benzene ring 55 Classify these alcohols as 1o, 2o, or 3o and name them: CH3 CH3

CH CH2OH Primary Alcohol 1o, 2-methyl-propan-1-ol CH3 CH3 CH3 Tertiary Alcohol 3o, t-butanol (or 2-methyl-propan-2-ol)

OH OH CH3 Secondary Alcohol 2o, phenol C OH CH CH2CH3

Secondary Alcohol 2o, butan-2-ol 56 Oxidation of Alcohols: Do I have a tertiary alcohol? Primary and Secondary alcohols can be oxidized (dehydrogenation) Primary alcohols are oxidized to the aldehyde and then the carboxylic acid

Secondary alcohols only oxidize to the ketone tertiary alcohols do not dehydrogenate (oxidize) + Cr3+ + Cr3+ If chromic acid is used for H2CrO4=[O] then the product includes Cr3+ which is green 57 Oxidation with Chromate Negative test

tertiary alcohol Positive test primary or secondary alcohol Test of a Phenol: FeCl3 test Phenols do not oxidize You can test for a phenol with ferric chloride (FeCl3) Phenol + Fe3+ [FeFe3+.phenol complex] colorless yellow purple

Acidity of Phenol In water, alcohols are neutral but phenol acts as a weak acid phenol phenoxide ion ALDEHYDES AND KETONES The carbonyl group:

Aldehydes have at least one hydrogen attached to the carbonyl group. Ketones have two carbons attached to the carbonyl group. NAMING ALDEHYDES Find the longest carbon chain that contains the aldehyde group. Change ending of the root hydrocarbon name by

dropping e and adding al. All other branches and groups are named and located using standard IUPAC system. Examples: 3-bromobutanal 2-ethylbutanal NAMING KETONES Find the longest chain that contains C=O. Using the root alkane name, drop the e ending and change to one.

Number the longest carbon chain so the C=O group has the lowest number. Name and number other substituents as before. Examples: 3-methyl-2-pentanone 2-methylcyclohexanone ALDEHYDE AND KETONE REACTIONS Recall the oxidation of alcohols to produce aldehydes and ketones:

ALDEHYDE AND KETONE REACTIONS (continued) The difference in reactivity toward oxidation is the chief reason why aldehydes and ketones are classified in separated families. ALDEHYDE AND KETONE REACTIONS The ease with which aldehydes are oxidized allows us to test for the presence of aldehydes with Tollens reagent or Benedicts reagent.

In general, ketones fail to react with these reagents. From left to right, three test tubes containing potassium dichromate (K2Cr2O7), acetone, and benzaldehyde. After the addition of equal amounts of K2Cr2O7, the acetone remains unreacted, whereas the benzaldehyde is oxidized. ALDEHYDE AND KETONE REACTIONS (continued)

In the presence of aldehydes, Tollens reagent produces a silver coating on glass. This method is used to produce mirrors and silver ornaments. ALDEHYDE AND KETONE REACTIONS (continued) Bradys Test for Aldehydes and Ketones 2,4-Dinitrophenylhydrazine can be used to qualitatively detect the carbonyl functionality of a ketone or aldehyde functional group. A positive test is signaled by a yellow, orange or red precipitate

RR'C=O + C6H3(NO2)2NHNH2 C6H3(NO2)2NHNCRR' + H2O alcohol, aldehyde, ketone or acid? Chromic Acid Test (1o or 2o alcohol and aldehyde only) 2,4-D Test (all ketones and aldehydes) Tollens Test (aldehyde only) Sodium Bicarbonate Test (carboxylic acid) Boiling Point

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