Download Structural Types and Properties of Alkanes, Alkenes, and Alkynes and more Lecture notes Applied Chemistry in PDF only on Docsity!
CHAPTER 2
HYDROCARBONS
A hydrocarbon molecule contains only carbon and hydrogen. Hydrocarbons are further divided into several structural types called, alkanes, alkenes, alkynes and aromatics. The first three types are presented in this chapter, and aromatics are presented in a later chapter.
2.1 Alkanes
2.1a Structure A chain of carbon atoms that contains the maximum number of hydrogen atoms has the general formula CnH2n+2 (n = number of carbon atoms). This type
of hydrocarbon is called an alkane. Alkanes can exist in straight chains, chains with branching, and cycles. The carbon atom in an alkane has four sp3 hybridized and forms four single bonds that are equidistant from each other. The shape is referred to as tetrahedral with a C-C bond length of 1.54 Å and bond angles of 109.5o.
H
HH H
1.54 A
109.5o
sp^3 hybrid tetrahedral H
H
H
H
Methane
2.1b Nomenclature Hydrocarbon Chains Knowledge about the names of alkanes is extremely important because the names of alkanes form the basis for the names of many other types of organic compounds. The names of the first ten alkanes are shown below. Alkane names always end with -ane.
Carbons Structure Name
20 Ch 2 Hydrocarbons
1 CH4 methane 2 CH3CH3 ethane 3 CH3CH2CH3 propane 4 CH3CH2CH2CH3 butane 5 C5HI2 pentane 6 C6HI4 hexane 7 C7H16 heptane 8 C8Hl8 octane 9 C9H20 nonane 10 C10H22 decane
Alkanes that contain four or more carbons can have different structures with the same formula that are known as structural isomers. The structural isomers of C4H10 and of C5H12 are shown below. The number of structural isomers increases with the number of carbon atoms. Butane, C4H10, has two structural isomers, pentane, C5H12, has three, but heptane, C7H14, has nine.
C 4 H 10 CH 3 CH 2 CH 2 CH 3 CH 3 CHCH 3
CH 3
butane isobutane
straight chain (^) branched chain
C 5 H 12 CH 3 CH 2 CH 2 CH 2 CH 3 CH 3 CHCH 2 CH 3
CH 3
CH 3 CCH 3
CH 3
CH 3
pentane (^) isopentane (^) neopentane A systematic system of nomenclature for organic compounds has been developed by the International Union of Pure and Applied Chemistry (IUPAC). Names will be developed throughout the following chapters as the various compounds are encountered. The parent IUPAC name of an alkane is derived from the length of the longest chain in the compound. The chain is named, as shown above, and the suffix -ane is added. If alkane substituents are attached to the chain, they are named from their chain length by dropping the -ane and adding -yl. These are referred to as alkyl
22 Ch 2 Hydrocarbons
must be separated by commas and followed by a hyphen.
CH 3 CCH 2 CHCH 2 CH 3
CH 3
CH 3
CH 3
C 1 -C 2 -C 3 -C 4 -C 5 -C 6
2,2,4-trimethyl C
C
C
hexane When the alkyl substituents are different, they are assigned the lowest numbers possible and named in alphabetical order. 3-Ethyl-2-methylhexane is the IUPAC name for the compound below.
C 1 -C 2 -C 3 -C 4 -C 5 -C 6
C
hexane
C-C
methyl
ethyl
CH 3 CHCHCH 2 CH 2 CH 3
CH 3
CH 2 CH 3
CH 3 CHCHCH 2 CH 2 CH 3
CH 3
CH 2 CH 3
correct incorrect The compound above could be given another name because it contains a hexane parent chain. The alkyl substituent would be 3-isopropyl or 3-(1- methylethyl)-. The IUPAC system uses the chain with the greater number of substituents, thus using the wrong numbering gives a name with only one substituent. In alphabetizing the alkyl groups the prefixes di-, tri-, tetra, etc., do not count, but the i of isopropyl does count. In the compound below the ethyl is named before methyl without regard to the di- in dimethyl. It is acceptable that the number 3 comes before the number 2 in the name; alphabetical order is all that counts.
2.1 Alkanes 23
CH 3 -C
CH 3
CH 3
CH-CH 2 -CH 3
CH 2 CH 3
C 1 -C 2 C 3 -C 4 -C 5
C
C C-C
2,2-dimethyl
ethyl
pentane 3-ethyl-2,2-dimethylpentane Cyclic Hydrocarbons Hydrocarbons can also exist with the carbon atoms forming a ring or cycle. The cycloalkanes are named by adding the prefix "cyclo" to the name of the alkane based on the number of carbon atoms. The condensed structure representation without C for carbon or H for hydrogen is convenient for writing cyclic alkanes.
H 2 C
H 2 C CH 2
H 2 C
C C
C
C
H 2 C
C
H 2
CH 2
C
cyclopropane (^) cyclobutane
cyclopentane
H 2
H 2 H 2
H 2 H 2
Alkyl groups attached to cycloalkanes are named alphabetically and given the lowest possible number.
C
H 2 C
C
H 2
CH 2
CH
CH 3
H 3 C
CH 3 CH 3
H 3 C
CH 3
1,1,2-trimethylcyclopentane Cycloalkanes may also contain more than one ring, and their names are prefixed by the number of rings present, such as bicyclo[2.1.0]pentane. This is a cyclopentane but two rings are present. The names can become very complicated,
2.1 Alkanes 25
2-methylheptadecane C 18 H 38
Tiger moth sex attractant 2.1d Physical Properties of Alkanes Alkanes contain no electronegative atoms and thus are non-polar molecules that are useful as solvents for a wide range of organic substances. The boiling points of alkanes depend on the molecular mass and branching of the chain. Some general observations on boiling points are that branching decreases the boiling point, adding a CH2 or CH3 group increases the bp by 20-30o, and forming a ring
increases the bp by 10-20o.
2.1d Reactions of Alkanes Alkanes are relatively inert. Most reactions require specialized catalysts for breaking the carbon-carbon bonds. Burning, oxidation of alkanes, is a very important reaction in using alkanes as fuel. The fuels used in engines require additives to improve the combustibility for smooth and efficient burning.
CH 3 CH 2 CH 2 CH 2 CH 3
pentane (^) cyclopentane bp 35o^ bp 50o
CH 3 CHCH 2 CH 3
CH 3
isopentane bp 28o
CH 3 CHCH 2 CH 2 CH 3 CH 3 CH 2 CH 2 CH 2 CH 2 CH 3
CH 3
2-methylpentane (^) hexane (^) cyclohexane bp 62o bp 69o^ bp 81o
26 Ch 2 Hydrocarbons
9 O 2 6 CO 2 6 H 2 O
∆H = 950 kcal/mole
2.2 Alkenes
2.2a Structure Hydrocarbons can contain carbon bound to another carbon by a double bond made up of a sigma (σ) bond and a pi (π) bond. Compounds with double bonds are called alkenes and are considered unsaturated compounds, and an older system of terms calls them olefins. The general structure of an alkene in a chain is CnH2n, two hydrogens less than an alkane.
The alkene carbon atoms are sp2 hybridized and all of the single bonds are sp2 orbitals overlapped with orbitals from hydrogen or carbon. The unused p- orbitals in this hybridization join by side-to-side overlap to form a bond called a π- bond. The π-bond prevents rotation in the system and allows geometrical cis and trans isomers discussed in the stereochemistry chapter. The alkene is a planar structure with a C=C bond length of 1.34 Å and a C-H bond length of 1.1 Å. All bond angles are 120o.
H H
H H
π-bond
120 o
120 o sp^2 hybrid^ planar
1.34 A
1.1 A
2.2b Nomenclature Alkenes are named from the corresponding alkane name by dropping the - ane and by adding -ene. The alkene function is a higher priority than an alkyl group and thus it is given the lowest number in the chain.
28 Ch 2 Hydrocarbons
CH 3 H 3 C
1,5-dimethylcyclopentene not 1,2-dimethylcyclopentene
H 3 C
H 3 C
3,3-dimethylcyclohexene not 1,1-dimethyl-2-cyclohexene 2.2c Naturally Occurring Alkenes Alkene structures are also found in some interesting natural sources. Limonene is a natural substance found in lemon oil, and turpentine, a paint thinner, is found in pine trees.
limonene found in lemon oil
H 3 C CH 3
CH 3
turpentine found in pine trees The compound shaped like a soccer ball is not found in nature but is obtained by passing an electrical arc through pure carbon electrodes. It is named after Buckminster Fuller who proposed such structures. Thus the name is fullerene, but the compound is sometimes referred to as a Bucky Ball. Bucky Ball has the formula C60 and thus contains no hydrogen atoms.
C 60 fullerene Bucky Ball
Low molecular weight alkenes are obtained by "cracking" or breaking down alkanes at high temperature with special catalysts. Ethene is obtained from ethane and some natural gas wells contain ethene, which is an important resource for the production of polyethylene used in plastic bags and toys.
2.2 Alkenes 29
2.2d Physical properties of alkenes Alkenes, like alkanes, are non-polar materials with generally lower boiling points than the corresponding alkane. Alkenes also have strong odors, sometimes pleasant, sometimes not.
CH 3 CH 2 CH 2 CH 2 CH 3 CH 2 =CHCH 2 CH 2 CH 3
CH 3 CH=CHCH 2 CH 3
pentane 1-pentene
2-pentene
bp 36^0 bp 30
o
bp 37o
CH 2 =CHCH 2 CH 2 CH 2 CH 3
CH 3 CH=CHCH 2 CH 2 CH 3
CH 3 CH 2 CH 2 CH 2 CH 2 CH 3
hexane (^) 1-hexene
2-hexene
bp 69o^ bp 64o
bp 68o
2.2e Preparation of Alkenes Alkenes are often prepared by removal of substituents from saturated precursors. Dehydration (loss of H2O) and dehydrohalogenation (loss of H and
halogen) are two common elimination procedures for the preparation of alkenes. The Wittig reaction, which involves an aldehyde or ketone with an organophosphorus reagent is an extremely important alkene producing reaction. The reactions shown below all have very different reaction mechanisms that will be be discussed in detail in later chapters.
2.2 Alkenes 31
CH 2 =CH 2
CH 3 CH=CH 2
CH 3 CH=CHCH 3
CH 3 C=CHCH 3
CH 3
Alkene
cis trans
∆ H kcal/mole 33 30
28 26
Heats of Hydrogenation
A very important general trend in the stabilities of alkenes is found in these observations. Alkenes with more alkyl substituents are more stable. That means that an alkene with four alkyl groups attached to the alkene is more stable than one with four hydrogens attached. Later, many reactions will be observed to produce alkenes on the basis of the stability of the alkene. The listing shows relative alkene stability, it does not mean that ethylene is unstable, just is less stable than the others.
C=C
R R
R R
C=C
R R
R H
C=C
R R
H H
C=C
R H
H R
C=C
R H
H H
C=C
H H
H H
> >^ >^ > >
most stable trans^ cis least stable Cyclic alkenes are nearly always cis, but in some cases with increasing ring size, unstable trans alkenes are known. trans-Cyclooctene, shown below, is an example of a strained cyclic trans alkene.
Electrophilic Addition An important class of reaction with alkenes involves addition of an electrophile to the double bond in a process called electrophilic addition. The electrophile (E+ ) reacts with the alkene to produce a positively charged intermediate called a carbocation. A nucleophile reacts with the carbocation to complete the two- step addition process.
32 Ch 2 Hydrocarbons
C=C
E+
C-C
Nu-
E
C-C
E
Nu E = electrophile Nu-^ = nucleophile of the reagent E-Nu
Unsymmetrical reagents follow a path in which the anion of the electrophilic reagent is bound to the more substituted carbon in the final product. This type of addition is called Markovnikov addition. Certain additions that do not occur by the electrophilic mechanism give products in which the opposite orientation is found, and are termed anti-Markovnikov additions. The orientation of the substituents in the product is a result of the reaction mechanism and will be discussed in detail in later chapters.
CH (^3) Markovnikov Addition
HBr CH 3 Br
Electrophilic Additions Symmetrical Additon CH (^3) Br 2 CH 3 Br Br Unsymmertical Additon
Trans Addition of Bromine Atoms
ionic
Additions to Alkenes
CH 3
Br
Addition
CH (^3) HBr-Peroxide (^) Anti-Markovnikov
Non-Electrophilic Addition
CH 3 1) BH 3
2) H 2 O 2
CH 3
H
OH
Hydroboration cis Addition of H and OH
34 Ch 2 Hydrocarbons
by enzyme catalysis (enzymes are protein molecules). The epoxidation of an alkene and the reaction of an alkene with a proton are common reactions of alkenes found later in the chapter on alcohols. Finally, the biochemical marvel continues, as three methyl groups are lost and the molecule undergoes some rearrangement to produce cholesterol. The loss of methyl groups is not a common occurrence in organic chemistry. CH 3
OH
O P O P O
O O
several steps O O
Dimethylallyl pyrophosphate
OOC-CH 2 -C-CH 2 -CH 2 OH
Mevalonate 5 Carbon Atoms
CH 3
CH 3 Cyclase enzyme
6 fold
Squalene
polymerization
30 Carbon Atoms
CH 3
O
H 3 C
Squalene 2,3-epoxide H+
many steps lose 3 methyl groups
CH 3
HO
CH 3
Cholesterol
2.3 Alkynes 35
2.3 Alkynes
2.3a Structure Alkynes, also called acetylenes, contain a structural unit in which carbon is bonded to another carbon with three bonds, C^ C_._ The double unsaturation gives rise to the general formula of CnH2n-2 for alkynes. The carbon atoms in an alkyne
are sp hybridized. The single bonds are made of sp hybridized carbon atoms overlapped with hydrogen or carbon. The remaining two unhybridized p-orbitals join in side-to-side overlap to make two orthogonal p-bonds. The overall alkyne structure is linear with a C-C bond length of 1.20 Å and C-H bond length of 1.08Å.
H H H C C H
π-bond
π-bond
1.08 A 1.20 A
180 o sp hybrid linear
2.3b Nomenclature The IUPAC system of nomenclature derives an alkyne name from the corresponding alkane. The -ane is replaced by -yne and a number is used to give the location of the triple bond. Ethyne, C2H2, is the first alkyne and is also
commonly known as acetylene. Alkynes are named by locating the parent name from the longest chain containing the triple bond. The alkyne is given the lowest possible number, and numbering proceeds across the triple bond. Substituents are given the lowest numbers possible and are named alphabetically.
H C (^) C CH 3
CH 3 C C CH 3
H C C H
CH 3 C C CH 2 CHCH 3
CH 3
ethyne acetylene
propyne
2-butyne 5-methyl-2-hexyne
2.3 Alkynes 37
CH 3 CH 2 CH 2 CH 2 CH 2 CH 3 CH 2 =CHCH 2 CH 2 CH 2 CH 3 hexane (^) 1-hexene
1-hexyne
bp 69o^ bp 64o
bp 72o
HC CH 2 CH 2 CH 2 CH 3
2.3f Reactions of Alkynes Addition of hydrogen to alkynes can be controlled to add two hydrogen atoms and give an alkene as a product, or four hydrogen atoms can be added to give an alkane. Special methods are required to add just two hydrogen atoms. A catalyst of palladium on calcium carbonate containing lead ions (Lindlar catalyst) allows addition of one mole of hydrogen to give the cis- alkene, meaning the two substituents are on the same side of the double bond. Reductions with sodium in ammonia give the trans-alkene, which contains the two substituents on opposite sides of the double bond. Complete reduction of acetylenes to the hydrocarbon occurs easily on hydrogenation with a number of catalysts including palladium on carbon. H 2 /Pd(CaCO 3 )
Pb
H 3 C (^) C C CH 3
Na, NH 3
H H
H 3 C CH 3
H CH 3
H 3 C H
CH 3 CH 2 CH 2 CH 3
H 2 / Pd on C
cis
trans
add 2 H
add 4 H
Alkynes react with a number of reagents to give addition products in a manner similar to alkenes. Those reactions will be discussed in the chapter on halogens.
2.4 Conjugated Alkenes
An organic molecule can possess more than one unsaturated system. When these systems are far apart from one another they behave as isolated functions. But
38 Ch 2 Hydrocarbons
when they are separated by a single bond they are conjugated, and they behave as a unit instead of isolated systems. The orbital picture shows the electronic interaction of the systems. Several structures are shown below that are conjugated.
O π-system
conjugated diene (^) α,β-unsaturated carbonyl
CH=CH 2
eneyne
CH 2 =C=CH 2
allene not conjugated Some interesting dienes called allenes, but infrequently encountered, have two double bonds attached to one carbon. These systems are not conjugated.
2.5 Summary
Hydrocarbons are a class of organic compounds that contain only hydrogen and carbon. Hydrocarbons are divided into classes called alkanes, alkenes and alkynes. Alkanes, CnH2n+2, are sp3 hybridized and have a tetrahedral shape.
Usually volatile and found abundantly in oil and natural gas, alkanes are used widely as fuels. The combustion of alkanes , reaction with oxygen, produces carbon dioxide and water. Alkanes are systematically named, IUPAC rules, by adding an ane suffix to the term that designates the number of carbon atoms. Alkenes, CnH2n, are sp2 hybridized and have a planar shape. Alkenes are
prepared by dehydration and dehydrohalogenation reactions. Alkene chemistry is dominated by reactions of the double bond called a -bond. The double bond undergoes reduction, oxidation, and addition reactions. Alkenes are named by replacing the alkane suffix, ane , with a new suffix ene. A number is assigned to the position of the double bond. Alkynes, C2nH2-2, are sp hybridized and are linear. Acetylene is well-
known for its use in welding torches. The triple bond of alkynes can be reduced with hydrogen, or oxidized by a variety of reagents. Alkynes are named by replacing the ane suffix of the alkane with the yne suffix. The triple bond location is
