What do alkenes contain

More than half of this ethylene goes into the manufacture of polyethylene, one of the most familiar plastics. Propylene is also an important industrial chemical. It is converted to plastics, isopropyl alcohol, and a variety of other products. Although there is only one alkene with the formula C 2 H 4 ethene and only one with the formula C 3 H 6 propene , there are several alkenes with the formula C 4 H 8. Just as there are cycloalkanes, there are cycloalkenes.

These compounds are named like alkenes, but with the prefix cyclo - attached to the beginning of the parent alkene name. Now place the methyl group on the third carbon atom and add enough hydrogen atoms to give each carbon atom a total of four bonds.

Briefly identify the important distinctions between a saturated hydrocarbon and an unsaturated hydrocarbon. Ethylene : A space-filling model of ethylene, the simplest alkene, showing its planar structure. Alkenes contain a double bond that is composed of one sigma and one pi bond between two carbon atoms. The sigma bond has similar properties to those found in alkanes, while the pi bond is more reactive. The carbon atoms in the double bond are sp 2 hybridized, forming a planar structure.

Rotation around the double bond is disfavored, so alkenes form fairly stable isomers depending on the positioning of substituents on the same cis or opposite trans sides of the double bond. These isomers are called diastereoisomers. Thermal cracking : The factory of the Shukhov cracking process by the great Russian engineer and scientist Vladimir Shukhov in In petroleum geology and chemistry, thermal cracking is the process whereby complex organic molecules such as kerogens or heavy hydrocarbons are broken down into simpler molecules such as light hydrocarbons, by the breaking of carbon-carbon bonds in the precursors.

The melting and boiling points of alkenes are determined by the regularity of the packing, or the closeness, of these molecules. Alkene isomers that can achieve more regular packing have higher melting and boiling points than molecules with the same molecular formula but weaker dispersion forces. Alkenes are non-polar, and they are both immiscible in water and less dense than water.

They are generally soluble in organic solvents. In addition, they do not conduct electricity. Alkenes are more reactive than their related alkanes due to the relative instability of the double bond. They are more likely to participate in a variety of reactions, including combustion, addition, hydrogenation, and halogenation reactions.

Alkenes can also be reacted, typically in the presence of a catalyst, to form polymers. Large amounts of ethylene are produced from natural gas via thermal cracking. It is an important raw material for the synthesis of a number of plastics.

Alkenes and alkynes are generally more reactive than alkanes due to the electron density available in their pi bonds.

In particular, these molecules can participate in a variety of addition reactions and can be used in polymer formation.

Unsaturated hydrocarbons can participate in a number of different addition reactions across their double or triple bonds. Addition reactions : Alkenes participate in a variety of addition reactions. These addition reactions include catalytic hydrogenation addition of H 2 , halogenation reaction with X 2 , where X is a halogen , and hydrohalogenation reaction with H-X, where X is a halogen , among others.

Alkenes undergo diverse cycloaddition reactions. Most notable is the Diels—Alder reaction with 1,3-dienes to give cyclohexenes. Diels-Alder reaction : Here, the reaction of 1,3-butadiene the diene reacts with ethylene the dienophile to produce cyclohexene. It is split into the H- and the -OH components. Similar to the hydrohalogenation reaction, the hydrogen adds first, as it carries the partial positive charge. In more complex molecules, hydrohalogenation and hydration reactions can lead the formation of more than one possible product.

For example, if 2-methylpropene [ CH 3 2 CCH 2 ] reacts with water to form the alcohol, two possible products can form, as shown below. However, the addition reaction is not random.

One of the products is the major product being produced in higher abundance while the other product is the minor product. This occurs because the carbocation intermediate that forms as the reaction proceeds is more stable when it is bonded to other carbon atoms, than when it is bonded with hydrogen atoms, as seen in the example below:. In each reaction, the reagent adds across the double bond.

Write the equation for each reaction. What is the principal difference in properties between alkenes and alkanes? How are they alike? If C 12 H 24 reacts with HBr in an addition reaction, what is the molecular formula of the product? Alkenes undergo addition reactions; alkanes do not. Both burn. Complete each equation.

In an elimination reaction a molecule loses a functional group, typically a halogen or an alcohol group, and a hydrogen atom from two adjacent carbon atoms to create an alkene structure. Elimination reactions are essentially the reverse reaction of the hydration and hydrohalogenation addition reactions.

Elimination reactions can also occur with the removal of water from alcohol. A rearrangement reaction is a specific organic reaction that causes the alteration of the structure to form an isomer. With alkene structures, rearrangement reactions often result in the conversion of a cis -isomer into the trans conformation. Due to the high reactivity of alkenes, they usually undergo addition reactions rather than substitutions reactions.

The exception is the benzene ring. The double-bonded structure of the benzene ring gives this molecule a resonance structure such that all of the carbon atoms in the ring share a continually rotating partial bond structure. Thus, the overall structure is very stable compared to other alkenes and benzene rings do not readily undergo addition reactions. They behave more similarly to alkane structure and lack chemical reactivity. One of the few types of reactions that a benzene ring will undergo is a substitution reaction.

Recall from Chapter 7 that in substitution reactions an atom or group of atoms is replaced by another atom or group of atoms. Halogenation is a common substitution reaction that occurs with benzene ring structures. In the diagram below, notice that the hydgrogen atom is substituted by one of the bromine atoms. A polymer is as different from its monomer as a long strand of spaghetti is from a tiny speck of flour.

For example, polyethylene, the familiar waxy material used to make plastic bags, is made from the monomer ethylene—a gas. Polyethene pellets that are produced in factories can be melted, formed into a giant bubble, and then made into a film that is used in packaging, consumer products, and food services. There are two general types of polymerization reactions: addition polymerization and condensation polymerization.

This section will focus on addition polymerization reactions. For more information about condensation polymerization, see Chapter 10 In addition polymerization, the monomers add to one another in such a way that the polymer contains all the atoms of the starting monomers.

Ethylene molecules are joined together in long chains. The polymerization can be represented by the reaction of a few monomer units:. The bond lines extending at the ends in the formula of the product indicate that the structure extends for many units in each direction. Notice that all the atoms—two carbon atoms and four hydrogen atoms—of each monomer molecule are incorporated into the polymer structure. Because displays such as the one above are cumbersome, the polymerization is often abbreviated as follows, where n is the number of repeating units:.

Structure from: Magmar Many natural materials—such as proteins, cellulose and starch, and complex silicate minerals—are polymers. Artificial fibers, films, plastics, semisolid resins, and rubbers are also polymers. More than half the compounds produced by the chemical industry are synthetic polymers.

Some common addition polymers are listed in Table 8. Note that all the monomers have carbon-to-carbon double bonds. Many polymers are mundane e. Medical Uses of Polymers An interesting use of polymers is the replacement of diseased, worn out, or missing parts in the body. For example, about a , hip joints and , knees are replaced in US hospitals each year. The artificial ball-and-socket hip joints are made of a special steel the ball and plastic the socket.

People crippled by arthritis or injuries gain freedom of movement and relief from pain. Patients with heart and circulatory problems can be helped by replacing worn out heart valves with parts based on synthetic polymers.

These are only a few of the many biomedical uses of polymers. The hip is much like a ball-and-socket joint, and total hip replacements mimic this with a metal ball that fits in a plastic cup. What is a monomer? What is a polymer? How do polymer molecules differ from the molecules we have discussed in earlier sections of this chapter? What is addition polymerization? What structural feature usually characterizes molecules used as monomers in addition polymerization? Monomers are small molecules that can be assembled into giant molecules referred to as polymers, which are much larger than the molecules we discussed earlier in this chapter.

In addition polymerization, the monomers add to one another in such a way that the polymer contains all the atoms of the starting monomers. C H Cl Write the condensed structural formula of the monomer from which Saran is formed. To ensure that you understand the material in this chapter, you should review the meanings of the bold terms in the following summary and ask yourself how they relate to the topics in the chapter.

Any hydrocarbon containing either a double or triple bond is an unsaturated hydrocarbon. Alkenes have a carbon-to-carbon double bond. The general formula for alkenes with one double bond is C n H 2 n.

Alkenes can be straight chain, branched chain, or cyclic. Simple alkenes often have common names, but all alkenes can be named by the system of the International Union of Pure and Applied Chemistry and have the ending -ene. Cis-trans isomers or geometric isomers are characterized by molecules that differ only in their configuration around a rigid part of the structure, such as a carbon—to-carbon double bond or a ring.

The molecule having two identical or closely related atoms or groups on the same side is the cis isomer ; the one having the two groups on opposite sides is the trans isomer. The physical properties of alkenes are quite similar to those of alkanes. Like other hydrocarbons, alkenes are insoluble in water but soluble in organic solvents.

More reactive than alkanes, alkenes undergo A ddition Reactions across the double bond. There are four types of addition reactions: Hydrogenation which involves adding H 2 across the double bond, Hydrohalogenation which involves adding hydrogen and a halogen Cl, Br, or I across the double bond, Halogenation which involves adding two halogen atoms Cl, Br, or I across the double bond, and Hydration which involves adding water as H and -OH across the double bond.

Alkenes also undergo addition polymerization , molecules joining together to form long-chain molecules. The reactant units are monomers , and the product is a polymer.

Alkenes can also be involved in Rearrangement Reactions that convert one compound into a related isomer. Rearranging cis to trans isomers are common rearrangement reactions. Elimination Reactions can regenerate alkene structures by the removal of water or dehydration of alkanes. Alkynes have a carbon-to-carbon triple bond.

The properties of alkynes are quite similar to those of alkenes. They are named much like alkenes but with the ending — yne. Aromatic compounds contain a cyclic hydrocarbon, benzene C 6 H 6 with alternating double-bonds. Due to resonance structures, the aromatic ring is extremely stable and does not undergo the typical reactions expected of alkenes. The electrons that might be fixed in three double bonds are instead delocalized over all six carbon atoms.

The main reaction aromatic compounds will undergo are substitution reactions. A polycyclic aromatic hydrocarbon PAH has fused benzene rings sharing a common side. Give the molecular formula for each compound. Describe a physiological effect of some PAHs.

What are some of the hazards associated with the use of benzene? What is wrong with each name? Following are line-angle formulas for three compounds. Draw the uncondensed structure for each. Text for this chapter has been adapted from the creative commons resources listed below, unless otherwise noted in the text. Please click here to download: CH Chapter 8 PDF file This text is published under creative commons licensing, for referencing and adaptation, please click here.

Opening Essay 8. Back to the Top 8. Mathematically, this can be indicated by the following general formulas: In an alkene, the double bond is shared by the two carbon atoms and does not involve the hydrogen atoms, although the condensed formula does not make this point obvious, ie the condensed formula for ethene is CH 2 CH 2. Thus, until you become more familiar the language of organic chemistry, it is often most useful to draw out line or partially-condensed structures, as shown below: Back to the Top 8.

Looking Closer: Environmental Note Alkenes occur widely in nature. Concept Review Exercises Briefly describe the physical properties of alkenes. Answers Alkenes have physical properties low boiling points, insoluble in water quite similar to those of their corresponding alkanes. Key Takeaway The physical properties of alkenes are much like those of the alkanes: their boiling points increase with increasing molar mass, and they are insoluble in water.

Exercises Without referring to a table or other reference, predict which member of each pair has the higher boiling point. Answer 1-pentene 3-nonene. Concept Review Exercises Briefly identify the important distinctions between a saturated hydrocarbon and an unsaturated hydrocarbon. Answers Unsaturated hydrocarbons have double or triple bonds and are quite reactive; saturated hydrocarbons have only single bonds and are rather unreactive. Key Takeaway Alkenes are hydrocarbons with a carbon-to-carbon double bond.

Back to the Top. Note Acetylene is used in oxyacetylene torches for cutting and welding metals. Concept Review Exercises Briefly identify the important differences between an alkene and an alkyne. Answers Alkenes have double bonds; alkynes have triple bonds. Key Takeaway Alkynes are hydrocarbons with carbon-to-carbon triple bonds and properties much like those of alkenes.

Exercises Draw the structure for each compound. Name each alkyne. Concept Review Exercises How do the typical reactions of benzene differ from those of the alkenes? Answers Benzene is rather unreactive toward addition reactions compared to an alkene.

Recognizing Aromatic Compounds Which compounds are aromatic? Skill-Building Exercise Which compounds are aromatic? Key Takeaway Aromatic hydrocarbons appear to be unsaturated, but they have a special type of bonding and do not undergo addition reactions. Back to the Top Polycyclic Aromatic Hydrocarbons Some common aromatic hydrocarbons consist of fused benzene rings—rings that share a common side. Biologically Important Compounds with Benzene Rings Substances containing the benzene ring are common in both animals and plants, although they are more abundant in the latter.

Note So far we have studied only aromatic compounds with carbon-containing rings. Worked Example Which compounds can exist as cis-trans geometric isomers? Skill-Building Exercise Which compounds can exist as cis-trans isomers?

Concept Review Exercises What are cis-trans geometric isomers? Answers Cis-trans isomers are compounds that have different configurations groups permanently in different places in space because of the presence of a rigid structure in their molecule. Key Takeaway Cis-trans geometric isomerism exists when there is restricted rotation in a molecule and there are two different groups on each carbon atom involved in the chemical bond.

Addition Reactions Most reactions that occur with alkenes are addition reactions. The reaction mechanism for a generic alkene addition equation using the molecule X-Y is shown below: Figure 8.

Key Takeaway: Addition reactions convert an alkene into an alkane by adding a molecule across the double bond. Hydrogenation In a Hydrogenation reaction, hydrogen H 2 is added across the double bond, converting an unsaturated molecule into a saturated molecule.

Halogenation In a Halogenation reaction group 7A elements the halogens are added across the double bond. Hydrohalogenation In Hydrohalogenation , alkenes react with molecules that contain one hydrogen and one halogen. Hydration Just like when your are feeling thirsty, the terms hydration and dehydration refer to water. Skill-Building Exercise Write the equation for each reaction. Concept Review Exercises What is the principal difference in properties between alkenes and alkanes?

Answers Alkenes undergo addition reactions; alkanes do not.