Post on 20-Jan-2017
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Chapter 10
An Introduction to Organic Chemistry
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10.1 The chemistry of carbon
Why are there so many carbon-based compounds?Carbon atoms form stable, covalent bonds with each other.Carbon atoms form stable bonds with other elements, such as oxygen, nitrogen, sulfur, and halogens.Carbon atoms form multiple bonds with:
other carbon atoms (double & triple);oxygen (double);nitrogen (double & triple).
Carbon atoms can be arranged in linear chains, branched chains, and cyclic structures.
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10.1 Organic/inorganic compound differences
• Bond type–Organics have covalent bonds
• Electron sharing–Inorganics usually have ionic bonds
• Electron transfer• Structure
–Organics• Molecules• Nonelectrolytes
–Inorganics• Three-dimensional crystal structures• Often water-soluble, dissociating into ions -electrolytes
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10.1 Organic/inorganic compound differences
• Melting point & boiling point–Organics have covalent bonds
• Intermolecular forces broken fairly easily–Inorganics usually have ionic bonds
• Ionic bonds require more energy to break• Water solubility
–Organics• Nonpolar, water insoluble
–Inorganics• Water-soluble, readily dissociate
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10.1 Functional groups
Groups of atoms that cause organic compounds to exhibit particular properties are called functional groups.
e.g., alcohols always have an –OH group.
We’ll be studying organic chemistry by functional group.
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10.1 Functional groups
Alkenes contain a double bond between adjacent carbon atoms.
Alkynes contain a triple bond.
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10.1 Functional groups
An alcohol contains the hydroxyl (-OH) functional group.
In an ether, an oxygen atom is bonded to two carbon atoms.
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10.1 Functional groups
An aldehyde contains a carbonyl group (C=O), which is a carbon atom with a double bond to an oxygen atom.
In a ketone, the carbon of the carbonyl group is attached to two other carbon atoms.
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10.1 Functional groups
Carboxylic acids contain the carboxyl group, which is a carbonyl group attached to a hydroxyl group.
An ester contains the carboxyl group between carbon atoms.
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10.1 Functional groups
In amines, the functional group is a nitrogen atom.
In amides, the hydroxyl group of a carboxylic acid is replaced by a nitrogen group.
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10.2 Alkanes
Hydrocarbons are compounds composed only of carbon and hydrogen.
Saturated hydrocarbons have only single bonds.Unsaturated hydrocarbons have one or more double and/or triple bonds.
Saturated hydrocarbons are calledalkanes if they are acyclic.cycloalkanes if they are cyclic.
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10.2 Sources of alkanes
Petroleum, a naturally occurring mixture of hydrocarbons, is the main source of alkanes.
Liquid petroleum is called crude oil.hydrocarbons with 5 or more carbons
Gaseous petroleum is called natural gas.mostly methane, with ethane, propane, and butane
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10.2 Sources of alkanes
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10.2 Physical properties of alkanes
Alkanes are insoluble in water.Water molecules are polar and can take part in hydrogen bonding.Alkanes are nonpolar.“Like dissolves like.”
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10.2 Physical properties of alkanes
Alkanes have lower boiling points for a given molecular weight than most other organic compounds.
Alkanes are nonpolar.
Molecules are only weakly held together by van der Waals attractions.
Because the attractions are weak, less energy is needed to separate molecules from each other into the gas phase.
Therefore, boiling points are lower.
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10.2 Physical properties of alkanes
The boiling points of alkanes rise as the chain length increases and fall as the chains become branched and more nearly spherical.
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10.2 Physical properties of alkanes
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10.2 Physical properties of alkanes
This is a good place to answer your first Journal question for this module.[Use tag “water”]
Table salt (NaCl) dissolves in water, but iodine (I2) does not to any appreciable extent. Explain why this is, in terms of the molecular properties of salt, water, and iodine.
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10.2 Writing structural formulas
Some definitions:Atoms bonded in a “straight” line are in a continuous chain.
A branched chain has some carbons branching off the longest continuous chain.
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10.2 Writing structural formulas
Some hints:First, attach the carbon atoms to each other in the correct configuration.Add hydrogen atoms until each carbon atom has four bonds.
Structural formula of C3H8
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10.2 Abbreviated structural formulas
Writing out every atom and bond individually is inconvenient.Abbreviated structural formulas list each carbon with its hydrogens, adjacent to other carbons it is bonded to.
ethanol and dimethyl ether
n-pentane, 2,2-dimethylpropane
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10.2 Abbreviated structural formulas
Line structuresuse no element symbols.show only carbon-carbon bonds.
Line structures forn-pentane2-methyl-butane2,2-dimethylpropane
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10.2 Structures of alkanes
All alkanes fit the same general molecular formula.
CnH2n+2where n = number of carbons
Normal alkanes (n-alkanes) have carbon chains that are unbranched.A group of alkanes that each has one more –CH2– group is called a homologous series.
Properties of the molecules are similar.Properties change gradually as carbon atoms are added.
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10.2 Structures of alkanes
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10.2 Structures of alkanes
Which of the following are alkanes?
C7H18
C7H16
C8H16
C27H56
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10.2 Alkyl groups
An alkyl group is an alkane with one hydrogen atom removed It is named by replacing the -ane of the alkane name with –ylMethane becomes a methyl group.
CH4 – H = -CH3
Ethane becomes an ethyl group.CH3CH3 – H = -CH2CH3
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10.2 Alkyl groups
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10.2 Alkyl groups
Alkyl groups are classified according to the number of carbons attached to the carbon atom that joins the alkyl group to a molecule.
A primary carbon (1o) is directly bonded to one other carbon.A secondary carbon (2o) is directly bonded to two other carbons.A tertiary carbon (3o) is directly bonded to three other carbons.A quaternary carbon (4o) is directly bonded to four other carbons.
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10.2 Alkyl groups10
.2 A
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es
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10.2 Nomenclature
Organic compounds were initially named after their source, use, and properties.There are now over 15 million organic compounds known!Clearly, it is necessary to have a systematic method for naming.
Every compound must have a unique name.Structure must be determinable from the name.The name must be determinable from the structure.
The system we use is called IUPAC.It was devised by the International Union of Pure and Applied Chemistry.
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10.2 IUPAC rules for naming alkanes
The –ane ending is used for all alkanes.Alkanes without branching are named by the number of carbon atoms.For branched alkanes, the root name is the one for the longest continuous chain of carbon atoms.Groups attached to the main chain are called substituents.
Saturated groups composed of carbon and hydrogen are called alkyl groups.Alkyl groups are named after the corresponding alkane, with –yl replacing –ane.
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10.2 IUPAC rules for naming alkanes
The main chain is numbered so the first substituent has the lowest possible number.
All substituent names are preceded by the number of the carbon they are attached to.If there are two or more identical substituents, there are
two or more numbers preceding the name, anda prefix (di-, tri-, tetra-, . . .) on the substituent name.
If two or more different substituents are present, they are listed alphabetically (ignoring any number prefix).The only punctuation used is
commas to separate numbers from each other.hyphens to separate letters from numbers.
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10.2 IUPAC rules for naming alkanes
Give the IUPAC names for the following compounds:
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10.2 Alkyl and halogen substituents
Alkyl groups are named by changing the –ane ending of alkanes to –yl.
methyl CH3-ethylCH3CH2-n-propylCH3CH2CH2-isopropyl (1-methylethyl) CH3CHCH3
butylCH3CH2CH2CH2-sec-butyl (1-methylpropyl) CH3CH2CH2CH2
isobutyl (2-methylpropyl) (CH3)2CHCH2-tert-butyl (1,1-dimethylethyl) (CH3)3C-
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10.2 Alkyl and halogen substituents
Halogen substituents are named by changing the –ine ending of the element to –o.
fluoro- F-chloro- Cl-bromo- Br-iodo- I-
R is a general symbol for an alkyl group.R-H is any alkane.R-Cl or R-F or R-Br or R-I stand for alkyl halides.
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10.2 Alkyl and halogen substituents
Give a IUPAC name for CH2ClF.
Write a formula for each compound. 1,1,1,-trichlorodecane3,3,5-trimethylpentane
Name the following compounds.CH3CH2CH(CH2CH3)CH2CH2CH3
CH3C(CH3)2CH(CH2CH3)CH2CH2CH3
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10.2 Use of the IUPAC rules
2-methylpentane(not 4-methylpentane)
3-methylhexane(not 2-ethylpentane or 4-methylhexane)
2,2-dimethylbutane(not 2,2-methylbutane or 2-dimethylbutane)
3-bromo-1-chlorobutane(not 1-chloro-3-bromobutane or 2-bromo-4-chlorobutane)
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10.2 Isomerism
If more than one arrangement is possible for a given molecular formula, the different arrangements are called isomers.Structural isomers have the same molecular formula but different structural formulas. Consider the molecular formula C3H7Cl.
isomer 1isomer 2
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10.2 Isomerism
There are 5 structural isomers of C6H14.
Draw the 5 isomers and name them.
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10.2 Isomerism
Although all 5 isomers have the same molecular formula, their physical properties vary.
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10.2 Isomerism
How about another Journal question?[Use tag “isomers”]
In this power point, you’ve seen data indicating that the five isomers of C6H14 have different physical properties. With reference to the slides on properties of alkanes, explain the relative melting and boiling points of these isomers. That is, look at the shapes of the molecules (the amount of branching) and explain how the shapes correlate with the melting and boiling points.
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10.3 Cycloalkanes
Cycloalkanes have two fewer hydrogens than the corresponding chain alkane.
hexane, C6H14
cyclohexane, C6H12
The general formula for a cycloalkane is CnH2n.
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10.3 Cycloalkanes
To name cycloalkanes, add the prefix cyclo- to the name of the corresponding alkane.
Place substituents in alphabetical order before the base name as for alkanes.
For multiple substituents, use the lowest possible set of numbers; a single substituent requires no number.
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10.3 Cycloalkanes
Cyclopropane
Cyclobutane
Cyclohexane
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10.3 Naming cycloalkanes
Name the two cycloalkanes shown.Parent chain
6 carbon ring, cyclohexane 5 carbon ring, cyclopentane
Substituent 1 chlorine atom, chloro- 1 methyl group, methyl-
chlorocyclohexane
methylcyclopentane
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10.3 cis-trans isomers in cycloalkanes
Atoms of an alkane can rotate freely around the carbon-carbon single bond having an unlimited number of arrangements.Rotation around the bonds in a cyclic structure is limited by the fact that all carbons in the ring are interlocked.Formation of cis-trans (geometric) isomers is a consequence of the lack of free rotation around bonds.Stereoisomers are molecules that have the same structural formulas and bonding patterns, but different arrangements of atoms in space. See “Videos with
Models” in “Powerpoints and Related Materials”
folder for Module 1
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10.3 cis-trans isomers in cycloalkanes
Two groups may be on the same side (cis) of the imagined plane of the cyclo-ring or they may be on the opposite side (trans).Geometric isomers do not readily interconvert, because this would involve bond breaking and re-formation.
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10.3 Practice with cycloalkanes
Name the following compounds:
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10.3 Practice with cycloalkanes
Draw structures for the following compounds.
trans-1,4-dimethylcyclooctane
cis-1,3-dichlorocyclohexane
cis-1-bromo-2-fluoro-cyclobutane
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10.4 Conformations of alkanes
Geometric isomers have the same number of atoms of each type, but are bonded together differently.
Bonds must be broken to change one geometric isomer into another.
Conformers are identical molecules that are arranged differently in space (rotation around single bonds).
An intact molecule can be changed from one conformation to another.
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10.4 Ethane and butane
In an eclipsed conformation, groups on the front and back carbons are aligned when we look down the bond.
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10.4 Ethane and butane
In a staggered conformation, groups on the front and back carbons are not aligned when we look down the bond.
See “Videos with Models” in “Powerpoints and Related Materials”
folder for Module 1
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10.4 Journal
Last Journal question for this Module[Use tag “butanes”]
Consider the molecules butane and cyclobutane. If two substituents are added to cyclobutane, they can be cis- or trans-. If two substituents are added to butane, these is no possibility of cis- and trans- isomers;however, butane has conformers, and cyclobutane does not.
Explain these two observations in terms of the structures of butane and cyclobutane, and the definitions of cis- and trans- isomers and conformers.
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10.4 Cyclohexane
Because of the 109.5o carbon-carbon bonds, a cyclohexane ring doesn’t lie flat, but is puckered.
“Boat” conformation: both ends puckered up.
“Chair” conformation: one end up, one end down.
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10.4 Cyclohexane
The chair conformation of cyclohexane is more stable than the boat conformation because the hydrogens are less crowded.
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10.4 Cyclohexane
On each carbon in the cyclohexane ring, one hydrogen is vertical to a plane through the ring. These are the axial hydrogens.
The second hydrogen on each carbon is the equatorial hydrogen.
See “Videos with Models” in
“Powerpoints and Related
Materials” folder for Module 1
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10.5 Reactions: Combustion
Alkanes and cycloalkanes react with oxygen to produce carbon dioxide and water.
This is an oxidation reaction……but we usually call it combustion.
What’s wrong with this equation for the combustion of methane?
CH4 + O2 CO2 + H2O
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10.5 Reactions: Combustion
Guidelines for balancing combustion reactions:Balance carbon using CO2 product molecules.Balance hydrogen using H2O product molecules.Balance oxygen using O2 reactant molecules.
Write a balanced chemical equation for the combustion of the following compounds:
pentanecyclopropane2-methylhexane
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10.5 Reactions: Halogenation
In a substitution reaction, one atom substitutes for another on a molecule.Halogenation is the replacement of a hydrogen on an alkane with a halogen atom.Since alkanes and cycloalkanes aren’t very reactive, heat or light must be present for halogenation to take place.
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10.5 Reactions: Halogenation
Terminology: If one halogen is added per molecule:monochlorinationmonobromination
If all the hydrogens in the molecule are not equivalent, a mixture of products will form.
What are all the possible monochlorination products of pentane?
1-chloropropane 2-chloropropane
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3-D, rotatable model 3-D, rotatable model
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10.5 Reactions: Halogenation
If the halogenation reaction is allowed to continue, more than one halogen may add to each molecule.
CH4 + ½ Cl2 CH3Cl + HCl
CH4 + Cl2 CH2Cl2 + 2 HCl
CH4 + 3/2 Cl2 CHCl3 + 3 HCl
CH4 + 2 Cl2 CCl4 + 4 HCl
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End-of-chapter notes
Summary of reactions for the class of compounds studied in the chapter
Know what reactions these compounds undergo.Know what the reactants and products are.Know what the conditions for reaction are.Be able to write a balanced equation for each reaction.
Key termsThese should all be familiar to you by the time we finish the chapter.Know definitions for these terms—you’ll need them in later chapters.
Summary for each section of the chapterIf you don’t understand everything in the chapter summaries, go back and figure out what it means!