---
title: "Sodium Borohydride | Organic Chemistry"
description: "Sodium borohydride is a mild reducing agent in Organic Chemistry that turns aldehydes and ketones into alcohols through hydride addition."
canonical: "https://fiveable.me/organic-chem/key-terms/sodium-borohydride"
type: "key-term"
subject: "Organic Chemistry"
unit: "Unit 18"
---

# Sodium Borohydride | Organic Chemistry

## Definition

Sodium borohydride (NaBH4) is a mild reducing agent in Organic Chemistry that donates hydride to carbonyl compounds, usually turning aldehydes and ketones into alcohols.

## What It Is

Sodium borohydride, written as NaBH4, is a hydride reducing agent you use in Organic Chemistry when you want to convert a carbonyl compound into an alcohol without overreacting with the rest of the molecule. Its most common job is reducing aldehydes to primary alcohols and ketones to secondary alcohols.

The reaction works because the borohydride ion carries hydride, H-, which behaves like a nucleophile. The carbonyl carbon in an aldehyde or ketone is electrophilic because the C=O bond is polarized toward oxygen. When hydride attacks that carbon, the pi bond breaks and you get an alkoxide ion. A later protonation step, usually from water, methanol, ethanol, or an acid workup, gives the alcohol.

That two-step pattern matters. NaBH4 does not usually deliver the final alcohol in one move, it first forms the alkoxide intermediate. If you are tracing a mechanism on a problem set, that intermediate is the clue that the carbonyl has been reduced rather than simply substituted or rearranged. The product type depends on the starting carbonyl: aldehydes become primary alcohols, ketones become secondary alcohols.

One reason NaBH4 shows up so often is selectivity. Compared with lithium aluminum hydride, it is milder and easier to handle in many lab settings. That means it can reduce many aldehydes and ketones while leaving functional groups like esters or carboxylic acids alone under typical conditions. So if a synthesis asks for a reduction that stops at the alcohol stage, NaBH4 is often the reagent of choice.

A common classroom example is acetone. If you treat acetone with NaBH4 and then add acid or water, the carbonyl carbon gains hydride and ends up as 2-propanol. The carbonyl is gone, the oxygen is now part of an OH group, and the oxidation state of that carbon has dropped. That before and after picture is the main thing to recognize: NaBH4 converts a C=O into C-OH by hydride addition and protonation.

## Why It Matters

Sodium borohydride shows up everywhere in Organic Chemistry because carbonyl reduction is one of the basic moves in synthesis. If you can look at a molecule and see where a carbonyl can become an alcohol, you can predict products, plan routes, and identify reagents from reaction arrows.

It also teaches a core mechanism pattern: nucleophilic addition to a carbonyl. That pattern comes up again with Grignard reagents and with other hydride donors, so NaBH4 is a good place to practice following electron flow. You are not just memorizing a reagent, you are learning how electrophilic carbonyl carbons behave.

The reagent also helps you compare functional-group reactivity. In many problems, the whole challenge is choosing a reducing agent that is strong enough to change the target carbonyl but mild enough to leave other groups unchanged. That makes NaBH4 a favorite in synthesis questions where selectivity matters.

If your class includes lab work, NaBH4 often appears in product isolation or reaction prediction questions, where you identify the alcohol product and the intermediate alkoxide. It is a small reagent with a big payoff because it connects mechanism, oxidation state changes, and synthesis design in one reaction.

## Connections

### Reducing Agent

NaBH4 is a specific reducing agent, which means it lowers the oxidation state of another compound by delivering hydride. In Organic Chemistry, this label helps you sort reactions by what kind of change they cause, not just by what reagent is written over the arrow. NaBH4 is considered mild compared with stronger hydride sources, so its reactivity limits matter.

### Carbonyl Compound

NaBH4 works on carbonyl compounds because the C=O bond makes the carbonyl carbon electrophilic. That is why aldehydes and ketones are the main substrates for NaBH4 reductions. If you can spot a carbonyl, you can usually predict whether a hydride reduction is possible and what alcohol product you should draw.

### Nucleophilic Addition

The first step of NaBH4 reduction is nucleophilic addition of hydride to the carbonyl carbon. This is the same core idea behind many carbonyl reactions in Organic Chemistry, where a nucleophile attacks an electrophilic carbon and an intermediate forms before workup. The alkoxide you get after attack is the intermediate you should look for.

### [Aldehyde Intermediate](/organic-chem/key-terms/aldehyde-intermediate)

In many reduction pathways, an aldehyde can be the point where a molecule is ready to be turned into a primary alcohol. NaBH4 readily reduces aldehydes, so spotting an aldehyde intermediate can tell you the next step in a synthesis or mechanism question. It is a common checkpoint in carbonyl chemistry.

## On the AP Exam

A quiz item or problem set question will usually show a starting carbonyl and ask for the product after NaBH4, or ask you to choose the right reagent for a selective reduction. Your job is to identify the carbonyl, decide whether it is an aldehyde or ketone, and draw the alcohol product after protonation. If the molecule has multiple functional groups, you also need to notice what NaBH4 leaves untouched. In mechanism questions, show the hydride attack first, then the alkoxide, then the protonation step.

## Sodium Borohydride vs Lithium Aluminum Hydride

NaBH4 and LiAlH4 both reduce carbonyls, but they are not interchangeable. NaBH4 is milder and is usually used for aldehydes and ketones, while LiAlH4 is much stronger and can reduce esters, carboxylic acids, and more. If a question asks for selective reduction, NaBH4 is often the safer choice.

## Key Takeaways

- Sodium borohydride is a mild hydride reducing agent that turns aldehydes into primary alcohols and ketones into secondary alcohols.
- The mechanism starts with hydride attack on the electrophilic carbonyl carbon, followed by protonation of the alkoxide intermediate.
- NaBH4 is often chosen when you want to reduce a carbonyl without affecting more resistant functional groups under typical conditions.
- If you see NaBH4 on a reaction arrow, think carbonyl reduction first, then ask what kind of alcohol should form after workup.
- The reagent is a good way to practice nucleophilic addition and oxidation state changes at the same time.

## FAQs

### What is sodium borohydride in Organic Chemistry?

Sodium borohydride (NaBH4) is a hydride reducing agent used to convert aldehydes and ketones into alcohols. It works by adding H- to the carbonyl carbon, then the intermediate is protonated to give the alcohol.

### What does NaBH4 reduce?

NaBH4 most commonly reduces aldehydes to primary alcohols and ketones to secondary alcohols. In a typical Organic Chemistry class, it is treated as a mild reagent, so it does not usually reduce less reactive groups like esters or carboxylic acids under normal conditions.

### How is sodium borohydride different from lithium aluminum hydride?

Both are hydride donors, but NaBH4 is milder and more selective. LiAlH4 is stronger and can reduce a wider range of carbonyl-containing compounds, which is why reagent choice matters in synthesis problems.

### What product do you get when acetone reacts with sodium borohydride?

Acetone is reduced to 2-propanol. The carbonyl carbon gains hydride, the oxygen is protonated, and the C=O becomes an alcohol group.

## Related Study Guides

- [18.2 Preparing Ethers](/organic-chem/unit-18/preparing-ethers/study-guide/3u7GZmbiWcwrO1FM)
- [17.4 Alcohols from Carbonyl Compounds: Reduction](/organic-chem/unit-17/alcohols-from-carbonyl-compounds-reduction/study-guide/X7RRN1fRDwwNl6rj)
- [17.3 Preparation of Alcohols: A Review](/organic-chem/unit-17/preparation-alcohols-review/study-guide/cJibOE9XyQSYW29v)
- [16.9 Reduction of Aromatic Compounds](/organic-chem/unit-16/reduction-aromatic-compounds/study-guide/qtHhEGHURHiKS9eJ)
- [19.7 Nucleophilic Addition of Hydride and Grignard Reagents: Alcohol Formation](/organic-chem/unit-19/nucleophilic-addition-hydride-grignard-reagents-alcohol-formation/study-guide/z1vVGHUw2ftP8JJD)

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