free radical substitution of propane - an unanswerable cape chemistry question related to unit 2...
DESCRIPTION
This presentation addresses the method used to calculate relative amounts of monosubstituted products as a result of free radical substitution involving alkanes and chlorine at RTP in the presence of sunlight/uv light. It also deals with a CAPE Unit 2 Chemistry multiple choice question on the topic where students were given no correct option to choose.TRANSCRIPT
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ONE CAPE MULTIPLE CHOICE QUESTION WITH NO POSSIBLE ANSWERWITH NO POSSIBLE ANSWER
FINDING THE RATIO OFFINDING THE RATIO OF
1‐CHLOROPROPANE
TO
2‐CHLOROPROPANE2 CHLOROPROPANEWHEN PROPANE REACTS WITH CHLORINE GAS IN THE PRESENCE OF SUNLIGHT
FORFOR
CAPE UNIT 2 CHEMISTRY STUDENTS
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Overview Of Content
1. Using butane to show that probability l d t di t th ti falone does not predict the ratio of monochlorinated products.
2. Discussion of factors which affect ratio of d tproducts.
3 Application of these factors to explain3. Application of these factors to explain experimental results of propane’s hl i ti
2
chlorination.Compiled by Denison at Global in Cunupia. 739-2656.http://www.facebook.com/CAPEChemistryLessons
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Overview Of Content
4. The CAPE question which has no answer.
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Concept Of Primary And Secondary
A primary carbon is one which is bonded to l th b tonly one other carbon atom.
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Concept Of Primary And Secondary
A primary carbon is one which is bonded to l th b tonly one other carbon atom.
A secondary carbon is one which is bonded to only two other carbon atoms.y
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Concept Of Primary And Secondary
A primary carbon is one which is bonded to l th b tonly one other carbon atom.
A secondary carbon is one which is bonded to only two other carbon atoms.y
(Tertiary carbons will not be encountered here and are thus not discussed.)
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Primary Carbons In Butane
Primary carbon atoms are circled above in red.
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Secondary Carbons In Butane
Secondary carbon atoms are circled above in green.
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Primary And Secondary Hydrogens
A i h d i hi h i b d dA primary hydrogen is one which is bonded to a primary carbon atom.
A secondary hydrogen is one which is bonded y y gto a secondary carbon atom.
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Primary Hydrogens In Butane
Primary hydrogen atoms are circled above in red.
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Secondary Hydrogens In Butane
Secondary hydrogen atoms are circled above in green.
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P i C H b d th b tPrimary C‐H bonds are those between a primary H and a primary C.
Secondary C‐H bonds are those between a secondary H and a secondary C.secondary H and a secondary C.
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Monochlorination Of Butane
Two different alkyl halides are obtained from th hl i ti f b tthe monochlorination of butane.
Substitution of a hydrogen bonded to one of the terminal carbons produces p1‐chlorobutane, whereas substitution of a hydrogen bonded to one of the internalhydrogen bonded to one of the internal carbons forms 2‐chlorobutane.
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The expected (statistical) distribution ofThe expected (statistical) distribution of products is:
60% 1‐chlorobutane
and
40% 2‐chlorobutane14
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Such a ratio is based on the id th tidea that:
i f b ’ h d bSix of butane’s 10 hydrogens can be substituted to form 1‐chlorobutane, whereas
only four can be substituted to form2‐chlorobutane.
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This assumes, however, that all of the C‐HThis assumes, however, that all of the C H bonds in butane are equally easy to break.
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This is an incorrectThis is an incorrect assumption!assumption!
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If this were the case then the relativeIf this were the case then, the relative amounts of the two products would depend
l h b bili f hl i di lonly on the probability of a chlorine radical colliding with a primary hydrogen, compared with its colliding with a secondary hydrogen.
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When we carry out the reaction in the laboratory and analyze the product howeverlaboratory and analyze the product, however, we find that it is:
29% 1‐chlorobutane
and
71% 2 chlorobutane71% 2‐chlorobutane.
Therefore, probability alone does not explain the ratio of products.
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Because more 2‐chlorobutane is obtained than expected:than expected:
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Because more 2‐chlorobutane is obtained than expected:than expected:
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Because more 2‐chlorobutane is obtained than expected:than expected:
we conclude that it must be easier to substitute a hydrogen atom of a secondary carbon than of a primary carbon.carbon than of a primary carbon.
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After experimentally determining the amount of each chlorination product obtainedof each chlorination product obtainedfrom various hydrocarbons, chemists were bl t l d th table to conclude that:
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1. At room temperature it is 5.0 times easier for a chlorine radical to abstract afor a chlorine radical to abstract a hydrogen atom from a tertiary
b th f i bcarbon than from a primary carbon,
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1. At room temperature it is 5.0 times easier for a chlorine radical to abstract afor a chlorine radical to abstract a hydrogen atom from a tertiary
b th f i bcarbon than from a primary carbon,
and
2. It is 3.8 times easier to abstract a hydrogen atom from a secondary carbonhydrogen atom from a secondary carbon than from a primary carbon.
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To determine the relative amounts of productsobtained from radical chlorination ofobtained from radical chlorination ofan alkane,
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To determine the relative amounts of productsobtained from radical chlorination ofobtained from radical chlorination ofan alkane,
both probability (the number of hydrogens that can be abstracted that will lead to the formation of the particular product) and p p )reactivity (the relative rate at which a particular hydrogen is abstracted)a particular hydrogen is abstracted)
must be taken into account.
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When both factors are considered,
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When both factors are considered,
the calculated amounts
of 1‐chlorobutane and 2‐chlorobutane
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When both factors are considered,
the calculated amounts
of 1‐chlorobutane and 2‐chlorobutane
agree with g
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When both factors are considered,
the calculated amounts
of 1‐chlorobutane and 2‐chlorobutane
agree with g
the amounts obtained experimentally.the amounts obtained experimentally.
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Propane – Primary And Secondary Carbons
There are two primary carbon atoms inThere are two primary carbon atoms in propane, circled in red above.
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Propane – Primary And Secondary Carbons
There is one secondary carbon atom inThere is one secondary carbon atom in propane, circled in green above.
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Propane – Primary And Secondary Hydrogens
There are SIX primary hydrogen atoms inThere are SIX primary hydrogen atoms in propane, circled in red above.
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Propane – Primary And Secondary Hydrogens
There are TWO secondary hydrogen atoms inThere are TWO secondary hydrogen atoms in propane, circled in green above.
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Keep the numbers 6 and 2 closeKeep the numbers 6 and 2 close to the front of your mind because ythey will come into play very soon in a calculation similar to the one
done for butane previouslydone for butane previously.
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Chlorination Of Propane
When propane undergoes chlorination, ti it diff b t d dreactivity differences between secondary and
primary hydrogens come into play.
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Monosubstituted Propanes
Looking only at monochlorinated products, we th t th t 1 hl dsee that there are two: 1‐chloropropane and
2‐chloropropane.
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If h i i f i d d C HIf the reactivity of primary and secondary C‐Hbonds were identical, then the statistical ratio of 1‐chloropropane to 2‐chloropropane should be 3:1 because there are 6 primary hydrogen p y y gatoms and 2 secondary hydrogen atoms.
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This is not soThis is not so.
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The actual ratio of 1‐chloropropane
toto 2‐chloropropanep p
(as determined by experiment)
is 1 : 1.2243Compiled by Denison at Global in Cunupia. 739-2656.
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It can be calculated as follows:It can be calculated as follows:
Relative amount of 1-chloropropane
Number of hydrogens × reactivityNumber of hydrogens × reactivity6 × 1 = 6
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It can be calculated as follows:It can be calculated as follows:
Relative amount of 1-chloropropane
Number of hydrogens × reactivity
Relative amount of 2-chloropropane
Number of hydrogens × reactivityNumber of hydrogens × reactivity6 × 1 = 6
Number of hydrogens × reactivity2 × 3.8 = 7.6
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It can be calculated as follows:It can be calculated as follows:
Relative amount of 1-chloropropane
Number of hydrogens × reactivity
Relative amount of 2-chloropropane
Number of hydrogens × reactivityNumber of hydrogens × reactivity6 × 1 = 6
Number of hydrogens × reactivity2 × 3.8 = 7.6
This gives a ratio of 1 : 1.27
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It can be calculated as follows:It can be calculated as follows:
Relative amount of 1-chloropropane
Number of hydrogens × reactivity
Relative amount of 2-chloropropane
Number of hydrogens × reactivityNumber of hydrogens × reactivity6 × 1 = 6
Number of hydrogens × reactivity2 × 3.8 = 7.6
This gives a ratio of 1 : 1.27
Which agrees closely with the experimentally determined ratio of 1 : 1 22
47
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h di h liThe preceding theory applies to free radical chlorination at RTP infree radical chlorination at RTP in the presence of sunlight/uv light.p g g
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O l h dOnly when temperatures exceed 300 °C do both primary and300 C do both primary and
secondary reactivities both take a yvalue of 1.
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The Un‐Answerable CAPE Question
2010 Q4
When propane reacts with Cl2(g) in the presence of sunlight, two possible monosubstituted products, 1‐chloropropane and 2‐chloropropane are formed. What is the ratio of 1‐chloropropane to 2‐chloropropane?
(A) 1 : 2(B) 1 3(B) 1 : 3(C) 2 : 1(D) 3 : 1
*I saw this question in a few places online during the recently concluded examination period and was told about it by students who wrote exams in 2010.
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examination period, and was told about it by students who wrote exams in 2010.
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As we saw previously, the ratio is 1 : 1.27, so the first number in the ratio must be 1 and the 2‐chloropropane must be present in a larger
amount Therefore C and D are outamount Therefore C and D are out.
2010 Q42010 Q4
Wh t ith Cl i th f li ht t iblWhen propane reacts with Cl2(g) in the presence of sunlight, two possible monosubstituted products, 1‐chloropropane and 2‐chloropropane are formed. What is the ratio of 1‐chloropropane to 2‐chloropropane?
(A) 1 : 2(B) 1 : 3(C) 2 : 1(C) 2 : 1(D) 3 : 1
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2010 Q42010 Q4
Wh t ith Cl i th f li ht t iblWhen propane reacts with Cl2(g) in the presence of sunlight, two possible monosubstituted products, 1‐chloropropane and 2‐chloropropane are formed. What is the ratio of 1‐chloropropane to 2‐chloropropane?
(A) 1 : 2(B) 1 : 3
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What now?
2010 Q42010 Q4
Wh t ith Cl i th f li ht t iblWhen propane reacts with Cl2(g) in the presence of sunlight, two possible monosubstituted products, 1‐chloropropane and 2‐chloropropane are formed. What is the ratio of 1‐chloropropane to 2‐chloropropane?
(A) 1 : 2(B) 1 : 3
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1 : 3 could never be obtained by rounding 1 : 1 22 or 1 : 1 27 so B is out1 : 1.22 or 1 : 1.27 so B is out.
2010 Q42010 Q4
Wh t ith Cl i th f li ht t iblWhen propane reacts with Cl2(g) in the presence of sunlight, two possible monosubstituted products, 1‐chloropropane and 2‐chloropropane are formed. What is the ratio of 1‐chloropropane to 2‐chloropropane?
(A) 1 : 2(B) 1 : 3
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2010 Q42010 Q4
Wh t ith Cl i th f li ht t iblWhen propane reacts with Cl2(g) in the presence of sunlight, two possible monosubstituted products, 1‐chloropropane and 2‐chloropropane are formed. What is the ratio of 1‐chloropropane to 2‐chloropropane?
(A) 1 : 2
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But this can’t be the answer either since 1 : 2 also cannot be obtained byalso cannot be obtained by rounding 1 : 1.22 or 1 : 1.27.
2010 Q42010 Q4
Wh t ith Cl i th f li ht t iblWhen propane reacts with Cl2(g) in the presence of sunlight, two possible monosubstituted products, 1‐chloropropane and 2‐chloropropane are formed. What is the ratio of 1‐chloropropane to 2‐chloropropane?
(A) 1 : 2
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What do we do?
2010 Q42010 Q4
Wh t ith Cl i th f li ht t iblWhen propane reacts with Cl2(g) in the presence of sunlight, two possible monosubstituted products, 1‐chloropropane and 2‐chloropropane are formed. What is the ratio of 1‐chloropropane to 2‐chloropropane?
(A) 1 : 2(B) 1 : 3(C) 2 : 1(C) 2 : 1(D) 3 : 1
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Choose D and assume that they wanted you to wrongly use only probability in calculatingto wrongly use only probability in calculating
the answer.2010 Q42010 Q4
Wh t ith Cl i th f li ht t iblWhen propane reacts with Cl2(g) in the presence of sunlight, two possible monosubstituted products, 1‐chloropropane and 2‐chloropropane are formed. What is the ratio of 1‐chloropropane to 2‐chloropropane?
(A) 1 : 2(B) 1 : 3(C) 2 : 1(C) 2 : 1(D) 3 : 1
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