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Chemistry module 4 > Chapter 13 alkenes > Flashcards

Chapter 13 alkenes Flashcards

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1
Q

What are alkenes

A
  • are unsaturate hydrocarbons
  • contain at least once carbon to carbon double bond
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2
Q

What is the general formula for alkenes

A

CnH2n

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3
Q

What alkenes do not have the general formula Cn2n

A
  • cyclic alkenes
  • alkenes with one or more double bonds
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4
Q

Describe the bonding in alkenes

A
  • each carbon atom of the doule bond three of the four electrons form sigma bonds one to the other carbon atom of the double bond and two electrons to the other atoms
  • leaves on electron on each carbon atom of the double bond not involved in sigma bonds
  • electron is in a p orbital
  • a pi bond is formed
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5
Q

What is a pi bond and how does it occur in alkenes

A
  • sideways overlap of two p orbitals
  • one form each carbon atom of the double bond
  • each carbon atom contributes one electron the electron pair in the pi bond
  • locks the two carbon atoms in position [reventing rotation around the double bond
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6
Q

Where is the pi electron density in alkenes

A
  • above and below the line joining the nuclei of the bonding atoms
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7
Q

Describe the shape around the double bond in alkenes

A
  • trigonal planar
  • three regions of electron density around each of the carbon atoms
  • the three regions repel each other as far apart as possible so the bond angle around each carbon atom is 120
  • all of the atoms are in the same place
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8
Q

What are stereoisomers

A
  • same structural formula but different arrangement of atoms in space
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9
Q

What is E/Z isomerism

A
  • around a double bond
  • rotation is restricted and groups attatched to each carbon atom are therefore fixed relative to each other
  • C=C double bond
  • different groups attatched to each carbon atom of the double bond
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10
Q

What is E isomerism

A
  • higher priority are on the opposite side of each carbon atom
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11
Q

What is z isomerism

A
  • high priority are on the same side of each carbon atom
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12
Q

What is cis-trans isomerism

A
  • name commonly used to describe a special case of E/Z isomerism
  • molecules must have a C=C bond
  • each carbon in the double bond must be attatched to two same groups
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13
Q

What is a cis isomer

A

z isomer

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14
Q

What is a trans isomer

A

E isomer

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15
Q

What are the Cahn Ingold prelog rules

A
  • if groups of higher priority are on the same side of the double bond the compound is a z isomer
  • if the groups of higher priority are diagonally placed across the double bond the compound is an E isomer
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16
Q

How do you assign highest priority in E/Z isomerism

A
  • higher priority higher atomic number
17
Q

Describe why alkenes are more reactive than alkanes

A
  • prescence of the pi bond
  • C=C is made of one sigma and one pi bond
  • pi electrons are more exposed than electron in sigma bond
  • a pi bond readily breaks and alkenes undergo addition reactions
  • C-C single bond has a higher bond enthalpy than C=C so pi bond is weaker than sigma bonds
18
Q

Describe the hydrogenation of alkenes

A
  • when an alkene is mixed with hydrogen and passed over a nickel catalyt at 423 K it forms an alkane
  • hydrogen is added across the double bond
19
Q

Describe the halogenation of alkenes

A
  • alkenes undergo a rapid addition reaction with the halogens chlorine and bromine at room temperature
20
Q

What is the test for unsaturation

A
  • reaction of alkenes and bromine identify is C=C bond is present
  • orange colour disappear and turns colourless with the addition of bromine to an unsaturated compound
21
Q

Describe the addition reaction of alkenes and hydrogen halides

A
  • react with gaseous hydrogen halids at room temperature to form haloalkanes
  • alkene is a liquid hydrogen halid is bubbled through
  • alkenes also react with hydrochloric acid and hydrobromic acid
  • add across double bond tend to form two different compouunds with one C=C double bond
22
Q

Describe the hydration reactions of alkenes

A
  • alcohols are formed when alkenes react with steam H2O (g) in the prescence of phosphoric acid catalut H3PO4
  • steam adds across the double bond
  • additioon reaction is used widely in industry to product ethanol from ethene
  • two possible products
23
Q

Describe the basics of electrophillic addition

A
  • double bond in an alkene represenets a region of high electron denisty because of the prescence of pi electrons
  • high electron density of the pi electrons attract electrophiles
  • electrophile is an atom or group of atoms that is attracted to an electron rich centre and accepts an electron pair
24
Q

What is an electrophile

A
  • usually a posiive ion or a molecule containing an atom with a partial positive charge
25
Q

What is Markiwoffs rule

A
  • primary, secondary or tertiary carbocations can form
  • positive charge is on a carbon atom at the end of a chain
  • secondary carbocation positive charge is on a carbon atom with two carbon chains attatched
26
Q

Describe carbocation stability

A
  • tertiary carbocations are most stable and primary carbocations are the least stable
  • linked to the electron donating ability of alkul groups
  • each alkyl groups donates and pushes electrons towards the positive charge of the carbocation
  • poitive charge is spread over the alkyl groups
  • more alkyl groups are attatched to the positivley charged carbon atom, more charge is spread out making ion more stable
27
Q

How are carbocations classified

A

number of alkyl groups attatched to the positivley charge carbon atoom

28
Q

What are polymers

A
  • extremeley large molecules fromed from many thousands of repeat units of smaller molecules known as monomers
29
Q

What is addition polymerisation and it conditions

A
  • unsaturated alkene molecules undergo this to produce long chains containing no double bonds
  • carried out at high temperatures, presssures using catalysts
30
Q

Why are polymers hard to break down

A
  • lack of reactivity
  • most are non biodegradable growing amount of polymer waster serious enivronemental effects
31
Q

Why is recyclung good and how does it work

A
  • reduced environmental impact by conerving finite fossil fuels as well as decreaing amount of waste going to landfill
  • discarded polumers have to be sorted into types
  • polymers are then chopped into flaskes, washed, dried and metlted and cut into pellets to make new products
32
Q

What is PVC recycling and how does it work

A
  • hazardous due to high chlroine content and additives
  • not sustainable and when burnt release hydrogen chlroide which is a corrosive gas
  • grinding PVC and reusing it to maufacture new products
  • new technology uses solvents to dissolve the polymer
  • high grade PVC then recovered by precipatition from solvent and solvent is used again
33
Q

How can waste polymers be used as fuels

A
  • some polymers are difficult to recycle
  • as derived from petroleum or natural gas high stored energy value
  • waste polymers can be incinerated to produce heat generating steam to drive a turbine producing electricity
34
Q

What is feedstock recycling

A
  • describes the chemical and thermal processes that can reclain monomers, gases or oil from waste polymers
  • produced from crude oil in refineries
  • used as raw material for the production of new polymers
  • major advantage of feedstock recycling i that able to handle unsorted and unwashed polymers
35
Q

What are bioplastics and it’s benefits

A
  • produced from plant starch,cellulose, plant oils and protiens
  • renewable and sustainable alternative to oil based products
  • use of bioplatic not only protects our environment but conserv our valuble oil reserves
36
Q

What are biodegradable polymers

A
  • broken down by microorganims into water carbon dioxide and biological compound
  • polumers usually made from starch or cellulose of contain additives that alter the structure of trditional pollumer so that microorganisms can break them down
  • leave no visible or toxic residues
37
Q

What are photodegradable polymers

A
  • contain bonds which are weakened by abrobing light start degradation
  • light absorbing additives can be used

Chemistry module 4 (7 decks)

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